Rename tamer_closure -> closure.
Remove gather_rendezvous::linked_closure.

Currently unused.
Block on closures, not rendezvous.

This is an internal change; the goal is to let us unblock a closure
without reference to the rendezvous on which it was blocked.
Introduce tamed_class.

Declare your class with tamer::tamed_class as a base type.
Then if you block in a method of that class, your closure will
be automatically garbage collected.