add task-local reference counted smart pointers

To provide a reference counted pointer type with deterministic
destruction once managed boxes are switched over to a garbage
collector. Unlike managed boxes, these can be moved instead of just
copied/cloned which is helpful for avoiding reference counts.

Needs #5601 to be fixed in order for safety to be provided without the current ugly workaround of making the pointers contain `Option<@()>` and `Option<@mut ()>` (which are just set to `None`).

@brson: r?