alloc: Add Borrow impls for &String, &mut String, &Vec, and &mut Vec

This patch addresses a small papercut, where the auto-ref/deref
does not work well with generic types. One small example of this
is:

```rust
use std::borrow::Borrow;

fn foo<T: Borrow<[usize]>>(x: T) {
    println!("{:?}", x.borrow());
}

fn main() {
    let x = vec![1, 2, 3];
    foo(&x);
}
```

Without this patch, rust will error with:

```
error[E0277]: the trait bound `&std::vec::Vec<{integer}>: std::borrow::Borrow<[usize]>` is not satisfied
 --> foo.rs:9:5
  |
9 |     foo(&x);
  |     ^^^ the trait `std::borrow::Borrow<[usize]>` is not implemented for `&std::vec::Vec<{integer}>`
  |
  = help: the following implementations were found:
            <std::vec::Vec<T> as std::borrow::Borrow<[T]>>
  = note: required by `foo`

error: aborting due to previous error
```

This forces users to use `x.as_slice()` to get the code to compile.
This patch then implements the following to cut down on unnecessary
line noise:

* `Borrow<str>` for `&String`, `&mut String`
* `BorrowMut<str>` for `String` and `&mut String`
* `Borrow<[T]>` for `&Vec<T>` and `&mut Vec<T>`
* `BorrowMut<[T]>` for `&mut Vec<T>`

src/liballoc/slice.rs

@@ -1830,13 +1830,34 @@ impl<T> Borrow<[T]> for Vec<T> {
     }
 }
 
+#[unstable(feature = "borrow_ref_vec", issue = "45808")]
+impl<'a, T> Borrow<[T]> for &'a Vec<T> {
+    fn borrow(&self) -> &[T] {
+        &self[..]
+    }
+}
+
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T> BorrowMut<[T]> for Vec<T> {
     fn borrow_mut(&mut self) -> &mut [T] {
         &mut self[..]
     }
 }
 
+#[unstable(feature = "borrow_ref_vec", issue = "45808")]
+impl<'a, T> Borrow<[T]> for &'a mut Vec<T> {
+    fn borrow(&self) -> &[T] {
+        &self[..]
+    }
+}
+
+#[unstable(feature = "borrow_ref_vec", issue = "45808")]
+impl<'a, T> BorrowMut<[T]> for &'a mut Vec<T> {
+    fn borrow_mut(&mut self) -> &mut [T] {
+        &mut self[..]
+    }
+}
+
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T: Clone> ToOwned for [T] {
     type Owned = Vec<T>;

src/liballoc/str.rs

@@ -47,7 +47,7 @@ use core::iter::FusedIterator;
 use std_unicode::str::{UnicodeStr, Utf16Encoder};
 
 use vec_deque::VecDeque;
-use borrow::{Borrow, ToOwned};
+use borrow::{Borrow, BorrowMut, ToOwned};
 use string::String;
 use std_unicode;
 use vec::Vec;
@@ -182,6 +182,38 @@ impl Borrow<str> for String {
     }
 }
 
+#[unstable(feature = "borrow_mut_string", issue = "45808")]
+impl<'a> BorrowMut<str> for String {
+    #[inline]
+    fn borrow_mut(&mut self) -> &mut str {
+        &mut self[..]
+    }
+}
+
+#[unstable(feature = "borrow_ref_string", issue = "45808")]
+impl<'a> Borrow<str> for &'a String {
+    #[inline]
+    fn borrow(&self) -> &str {
+        &self[..]
+    }
+}
+
+#[unstable(feature = "borrow_ref_string", issue = "45808")]
+impl<'a> Borrow<str> for &'a mut String {
+    #[inline]
+    fn borrow(&self) -> &str {
+        &self[..]
+    }
+}
+
+#[unstable(feature = "borrow_ref_string", issue = "45808")]
+impl<'a> BorrowMut<str> for &'a mut String {
+    #[inline]
+    fn borrow_mut(&mut self) -> &mut str {
+        &mut self[..]
+    }
+}
+
 #[stable(feature = "rust1", since = "1.0.0")]
 impl ToOwned for str {
     type Owned = String;