Avoid the re-allocation of Activations the easy way: cache a few.
This shows the expected big win on controlflow-recursive, but a slowdown on an another benchmark; investigating.

Also fix a bug in not doing stack free right on detach activations.

An alternative would be to do use a non-GC heap cache, along with deferred tearoffs --- only doing them after the function returns. One would still have to make sure the activations don't move (not hard), but then the marking would require a special case.