REPR at_key_[boxed|ref] become at_key.

We pass the result register and also the kind down. This brings at_key
in line with at_pos. Some cleanups along the way, and MVMContext gets
to start working with natives as a side-effect.
Toss bind_key_ref.

Will use the generalized bind_key in its favor.
Generalize bind_key_boxed to bind_key.

Brings it in line with the way bind_pos works.