Reimplement Self in impls in the compiler

[nrc]

Currently the Self:: shorthand in the body of an impl is implemented by syntactic expansion in libsyntax. That leads to a bug if another item shadows the concrete type. E.g.,

struct Foo;

impl Foo {
    fn foo() {}
    fn bar() {
        let Foo = ...;
        Self::foo(); // Whoops!
    }
}

The Self on the commented line will cause an error in name resolution, since Self ends up referring to the local variable Foo, not the self type.