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 #20496.

src/librustc/middle/cfg/construct.rs

@@ -119,7 +119,7 @@ impl<'a, 'tcx> CFGBuilder<'a, 'tcx> {
             }
 
             ast::PatBox(ref subpat) |
-            ast::PatRegion(ref subpat) |
+            ast::PatRegion(ref subpat, _) |
             ast::PatIdent(_, _, Some(ref subpat)) => {
                 let subpat_exit = self.pat(&**subpat, pred);
                 self.add_node(pat.id, &[subpat_exit])