Minor cleanups: doc fixes, and clippy suggestions

Test that this is compatible back to Rust 1.26.0.

Rust 1.24.0 and earlier fail in the doctest due to
rust-lang/rust#33731

README.md

@@ -12,7 +12,7 @@ while the others will fall back to iterating and collecting.
 
 ## Example
 
-```
+```rust
 extern crate map_in_place;
 use map_in_place::MapVecInPlace;
 fn main() {
@@ -26,11 +26,11 @@ fn main() {
 }
 ```
 
-## Why all those restrictions?  
-The rust allocation interface is a bit more complex than the standard C one of
+## Why all those restrictions?
+The Rust allocation interface is a bit more complex than the standard C one of
 `malloc(size_t)` and `free(void*)`:
 
-First, malloc and free takes the alignment of the types you want to store,
+First, `alloc()` and `dealloc()` takes the alignment of the types you want to store,
 and allocating with one alignment and freeing with another is undefined behaviour.
 
 Second, Rust requires the owner to know the size of the memory to free,

lib.rs

@@ -14,15 +14,14 @@ I might also change the panic messages.
 ```
 extern crate map_in_place;
 use map_in_place::MapVecInPlace;
-fn main() {
-    let v = vec![8_u32,29,14,5];
-    let v = v.filter_map(|n| if n < 10 {Some( (n as u8+b'0') as char)}
-                             else      {None}
-                        );// happens in place
-    assert_eq!(&v, &['8','5']);
-    let v = v.map(|c| c as u8);// falls back to iterators
-    assert_eq!(&v[..], &b"85"[..]);
-}
+
+let v = vec![8_u32,29,14,5];
+let v = v.filter_map(|n| if n < 10 {Some( (n as u8+b'0') as char)}
+                         else      {None}
+                    );// happens in place
+assert_eq!(&v, &['8','5']);
+let v = v.map(|c| c as u8);// falls back to iterators
+assert_eq!(&v[..], &b"85"[..]);
 ```
 */
 
@@ -33,10 +32,10 @@ use std::{mem, ptr};
 
 
 // Error messages used by {map,retain}_in_place().
-static ERR_ZERO_SIZED: &'static str = "Cannot reuse memory for zero-sized types";
-static ERR_ALIGNMENT: &'static str = "`A` and `B` have different alignment";
-static ERR_NEED_EXACT_SIZE: &'static str = "`A` and `B` have different sizes";
-static ERR_NEED_DIVISIBLE_SIZE: &'static str
+static ERR_ZERO_SIZED: &str = "Cannot reuse memory for zero-sized types";
+static ERR_ALIGNMENT: &str = "`A` and `B` have different alignment";
+static ERR_NEED_EXACT_SIZE: &str = "`A` and `B` have different sizes";
+static ERR_NEED_DIVISIBLE_SIZE: &str
      = "The size of `A` is not equal to or a multiple of the size of `B`";