rustc: Update how Gc<T> is recognized

This commit uses the same trick as ~/Box to map Gc<T> to @T internally inside
the compiler. This moves a number of implementations of traits to the `gc`
module in the standard library.

This removes functions such as `Gc::new`, `Gc::borrow`, and `Gc::ptr_eq` in
favor of the more modern equivalents, `box(GC)`, `Deref`, and pointer equality.

The Gc pointer itself should be much more useful now, and subsequent commits
will move the compiler away from @T towards Gc<T>

[breaking-change]
syntax: Move the AST from @T to Gc<T>
rustc: Move the AST from @T to Gc<T>