Change & pat to only work with &T, and &mut with &mut T.

This implements RFC 179 by making the pattern `&<pat>` require matching
against a variable of type `&T`, and introducing the pattern `&mut
<pat>` which only works with variables of type `&mut T`.

The pattern `&mut x` currently parses as `&(mut x)` i.e. a pattern match
through a `&T` or a `&mut T` that binds the variable `x` to have type
`T` and to be mutable. This should be rewritten as follows, for example,

    for &mut x in slice.iter() {

becomes

    for &x in slice.iter() {
        let mut x = x;

Due to this, this is a

[breaking-change]

Closes rust-lang#20496.

Author: huonw

reference.md

@@ -3484,8 +3484,9 @@ fn main() {
 
 ```
 
-Patterns can also dereference pointers by using the `&`, `box` symbols,
-as appropriate. For example, these two matches on `x: &int` are equivalent:
+Patterns can also dereference pointers by using the `&`, `&mut` and `box`
+symbols, as appropriate. For example, these two matches on `x: &int` are
+equivalent:
 
 ```
 # let x = &3i;