runtime: remove r5js.ProcCallLike#{get,set}Continuation.
runtime: remove continuation_ fields from r5js.{Branch,ProcCall}.
runtime: remove r5js.Continuation#getNextContinuable.
runtime: remove r5js.Continuation#setNextContinuable in favor of ctor injection.
runtime: clean up call/cc impl.
runtime+misc: clean up no-longer-needed dependencies on r5js.Continuation.