std: Stabilize the fs module

This commit performs a stabilization pass over the `std::fs` module now that
it's had some time to bake. The change was largely just adding `#[stable]` tags,
but there are a few APIs that remain `#[unstable]`.

The following apis are now marked `#[stable]`:

* `std::fs` (the name)
* `File`
* `Metadata`
* `ReadDir`
* `DirEntry`
* `OpenOptions`
* `Permissions`
* `File::{open, create}`
* `File::{sync_all, sync_data}`
* `File::set_len`
* `File::metadata`
* Trait implementations for `File` and `&File`
* `OpenOptions::new`
* `OpenOptions::{read, write, append, truncate, create}`
* `OpenOptions::open` - this function was modified, however, to not attempt to
  reject cross-platform openings of directories. This means that some platforms
  will succeed in opening a directory and others will fail.
* `Metadata::{is_dir, is_file, len, permissions}`
* `Permissions::{readonly, set_readonly}`
* `Iterator for ReadDir`
* `DirEntry::path`
* `remove_file` - like with `OpenOptions::open`, the extra windows code to
  remove a readonly file has been removed. This means that removing a readonly
  file will succeed on some platforms but fail on others.
* `metadata`
* `rename`
* `copy`
* `hard_link`
* `soft_link`
* `read_link`
* `create_dir`
* `create_dir_all`
* `remove_dir`
* `remove_dir_all`
* `read_dir`

The following apis remain `#[unstable]`.

* `WalkDir` and `walk` - there are many methods by which a directory walk can be
  constructed, and it's unclear whether the current semantics are the right
  ones. For example symlinks are not handled super well currently. This is now
  behind a new `fs_walk` feature.
* `File::path` - this is an extra abstraction which the standard library
  provides on top of what the system offers and it's unclear whether we should
  be doing so. This is now behind a new `file_path` feature.
* `Metadata::{accessed, modified}` - we do not currently have a good
  abstraction for a moment in time which is what these APIs should likely be
  returning, so these remain `#[unstable]` for now. These are now behind a new
  `fs_time` feature
* `set_file_times` - like with `Metadata::accessed`, we do not currently have
  the appropriate abstraction for the arguments here so this API remains
  unstable behind the `fs_time` feature gate.
* `PathExt` - the precise set of methods on this trait may change over time and
  some methods may be removed. This API remains unstable behind the `path_ext`
  feature gate.
* `set_permissions` - we may wish to expose a more granular ability to set the
  permissions on a file instead of just a blanket "set all permissions" method.
  This function remains behind the `fs` feature.

The following apis are now `#[deprecated]`

* The `TempDir` type is now entirely deprecated and is [located on
  crates.io][tempdir] as the `tempdir` crate with [its source][github] at
  rust-lang/tempdir.

[tempdir]: https://crates.io/crates/tempdir
[github]: https://github.com/rust-lang/tempdir

The stability of some of these APIs has been questioned over the past few weeks
in using these APIs, and it is intentional that the majority of APIs here are
marked `#[stable]`. The `std::fs` module has a lot of room to grow and the
material is [being tracked in a RFC issue][rfc-issue].

[rfc-issue]: rust-lang/rfcs#939

[breaking-change]

lib.rs

@@ -39,7 +39,6 @@
 #![feature(int_uint)]
 #![feature(old_io)]
 #![feature(path)]
-#![feature(fs)]
 #![feature(rustc_private)]
 #![feature(staged_api)]
 #![feature(std_misc)]