Mutable reference mode for first arg to methods ignored due to self type parsing bug

[msullivan]

The following code:

enum thing { thing(int) }
impl thing {
    fn baz(&x: int) { x = 5 }
}
fn main() {}

fails to compile with the error:

nubs/method-mutable-ref.rs:3:22: 3:23 error: assigning to argument
nubs/method-mutable-ref.rs:3     fn baz(&x: int) { x = 5 }

which is complete nonsense, since it is a mutable argument. This is because a bug in the parser code that handles explicit self types causes the '&' to get dropped.