add an initial `Gc<T>` stub with the API
auto merge of #10576 : thestinger/rust/gc, r=pcwalton

This isn't very useful yet, but it does replace most functionality of `@T`. The `Mut<T>` type will make it unnecessary to have a `GcMut<T>` so I haven't included one. Obviously it doesn't work for trait objects but that needs to be figured out for `Rc<T>` too.