Following up from rust-lang#283
* Address all remaining clippy issues
* Switch `assert!(a==b)` to `assert_eq!(a, b)`
* `MAX` instead of `max_value()`

> warning: this seems like a manual implementation of the non-exhaustive pattern
>    --> src/mem.rs:46:1
>     |
>  46 |   pub enum FlushCompress {
>     |   ^---------------------
>     |   |
>     |  _help: add the attribute: `#[non_exhaustive] pub enum FlushCompress`
>     | |
>  47 | |     /// A typical parameter for passing to compression/decompression functions,
>  48 | |     /// this indicates that the underlying stream to decide how much data to
>  49 | |     /// accumulate before producing output in order to maximize compression.
>  ...  |
>  85 | |     _Nonexhaustive,
>  86 | | }
>     | |_^
>     |
>     = note: `#[warn(clippy::manual_non_exhaustive)]` on by default
>  help: remove this variant
>    --> src/mem.rs:85:5
>     |
>  85 |     _Nonexhaustive,
>     |     ^^^^^^^^^^^^^^
>     = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#manual_non_exhaustive
>
>  warning: this seems like a manual implementation of the non-exhaustive pattern
>     --> src/mem.rs:91:1
>      |
>  91  |   pub enum FlushDecompress {
>      |   ^-----------------------
>      |   |
>      |  _help: add the attribute: `#[non_exhaustive] pub enum FlushDecompress`
>      | |
>  92  | |     /// A typical parameter for passing to compression/decompression functions,
>  93  | |     /// this indicates that the underlying stream to decide how much data to
>  94  | |     /// accumulate before producing output in order to maximize compression.
>  ...   |
>  113 | |     _Nonexhaustive,
>  114 | | }
>      | |_^
>      |
>  help: remove this variant
>     --> src/mem.rs:113:5
>      |
>  113 |     _Nonexhaustive,
>      |     ^^^^^^^^^^^^^^
>      = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#manual_non_exhaustive
>
>  warning: the operation is ineffective. Consider reducing it to `(header[4] as u32)`
>    --> src/gz/bufread.rs:58:39
>     |
>  58 |                 r.part.header.mtime = ((header[4] as u32) << 0)
>     |                                       ^^^^^^^^^^^^^^^^^^^^^^^^^
>     |
>     = note: `#[warn(clippy::identity_op)]` on by default
>     = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#identity_op
>
>  warning: `ref` on an entire `let` pattern is discouraged, take a reference with `&` instead
>     --> src/gz/bufread.rs:197:13
>      |
>  197 |         let ref arr = [
>      |             ^^^^^^^
>      |
>      = note: `#[warn(clippy::toplevel_ref_arg)]` on by default
>      = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#toplevel_ref_arg
>  help: try
>      |
>  197 ~         let arr = &[
>  198 +             (crc.sum() >> 0) as u8,
>  199 +             (crc.sum() >> 8) as u8,
>  200 +             (crc.sum() >> 16) as u8,
>  201 +             (crc.sum() >> 24) as u8,
>  202 +             (crc.amount() >> 0) as u8,
>    ...
>
>  warning: the operation is ineffective. Consider reducing it to `crc.sum()`
>     --> src/gz/bufread.rs:198:13
>      |
>  198 |             (crc.sum() >> 0) as u8,
>      |             ^^^^^^^^^^^^^^^^
>      |
>      = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#identity_op
>
>  warning: the operation is ineffective. Consider reducing it to `crc.amount()`
>     --> src/gz/bufread.rs:202:13
>      |
>  202 |             (crc.amount() >> 0) as u8,
>      |             ^^^^^^^^^^^^^^^^^^^
>      |
>      = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#identity_op
>
>  warning: the operation is ineffective. Consider reducing it to `(buf[0] as u32)`
>     --> src/gz/bufread.rs:233:15
>      |
>  233 |     let crc = ((buf[0] as u32) << 0)
>      |               ^^^^^^^^^^^^^^^^^^^^^^
>      |
>      = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#identity_op
>
>  warning: the operation is ineffective. Consider reducing it to `(buf[4] as u32)`
>     --> src/gz/bufread.rs:237:15
>      |
>  237 |     let amt = ((buf[4] as u32) << 0)
>      |               ^^^^^^^^^^^^^^^^^^^^^^
>      |
>      = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#identity_op
>
>  warning: the operation is ineffective. Consider reducing it to `sum`
>     --> src/gz/write.rs:102:17
>      |
>  102 |                 (sum >> 0) as u8,
>      |                 ^^^^^^^^^^
>      |
>      = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#identity_op
>
>  warning: the operation is ineffective. Consider reducing it to `amt`
>     --> src/gz/write.rs:106:17
>      |
>  106 |                 (amt >> 0) as u8,
>      |                 ^^^^^^^^^^
>      |
>      = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#identity_op
>
>  warning: the operation is ineffective. Consider reducing it to `(self.crc_bytes[0] as u32)`
>     --> src/gz/write.rs:307:19
>      |
>  307 |         let crc = ((self.crc_bytes[0] as u32) << 0)
>      |                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>      |
>      = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#identity_op
>
>  warning: the operation is ineffective. Consider reducing it to `(self.crc_bytes[4] as u32)`
>     --> src/gz/write.rs:311:19
>      |
>  311 |         let amt = ((self.crc_bytes[4] as u32) << 0)
>      |                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>      |
>      = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#identity_op
>
>  warning: the operation is ineffective. Consider reducing it to `v.len()`
>     --> src/gz/mod.rs:215:25
>      |
>  215 |             header.push((v.len() >> 0) as u8);
>      |                         ^^^^^^^^^^^^^^
>      |
>      = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#identity_op
>
>  warning: you are using an explicit closure for copying elements
>     --> src/gz/mod.rs:221:27
>      |
>  221 |             header.extend(filename.as_bytes_with_nul().iter().map(|x| *x));
>      |                           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider calling the dedicated `copied` method: `filename.as_bytes_with_nul().iter().copied()`
>      |
>      = note: `#[warn(clippy::map_clone)]` on by default
>      = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#map_clone
>
>  warning: you are using an explicit closure for copying elements
>     --> src/gz/mod.rs:225:27
>      |
>  225 |             header.extend(comment.as_bytes_with_nul().iter().map(|x| *x));
>      |                           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider calling the dedicated `copied` method: `comment.as_bytes_with_nul().iter().copied()`
>      |
>      = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#map_clone
>
>  warning: the operation is ineffective. Consider reducing it to `mtime`
>     --> src/gz/mod.rs:231:21
>      |
>  231 |         header[4] = (mtime >> 0) as u8;
>      |                     ^^^^^^^^^^^^
>      |
>      = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#identity_op
>
>  warning: use of `offset` with a `usize` casted to an `isize`
>     --> src/mem.rs:365:27
>      |
>  365 |                 let ptr = output.as_mut_ptr().offset(len as isize);
>      |                           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `output.as_mut_ptr().add(len)`
>      |
>      = note: `#[warn(clippy::ptr_offset_with_cast)]` on by default
>      = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#ptr_offset_with_cast
>
>  warning: use of `offset` with a `usize` casted to an `isize`
>     --> src/mem.rs:506:27
>      |
>  506 |                 let ptr = output.as_mut_ptr().offset(len as isize);
>      |                           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `output.as_mut_ptr().add(len)`
>      |
>      = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#ptr_offset_with_cast
>
>  warning: `flate2` (lib) generated 18 warnings
>      Finished dev [unoptimized + debuginfo] target(s) in 0.02s