runtime: add r5js.ProcCallLike interface abstracting r5js.{Branch_,ProcCall}.
runtime: r5js.{Branch_,ProcCall} should implement r5js.ProcCallLike.
runtime: add r5js.ProcCallLike#{get,set}Continuation + stub impls.
runtime: push r5js.Continuable#continuation_ into subtype.
runtime+misc: remove r5js.Continuable#getContinuation.
runtime: remove unused r5js.Continuable.lastContinuable_.
runtime+macro: remove r5js.Continuable#setContinuation.
runtime+misc: r5js.Continuable#setStartingEnv should return nothing.
runtime: add r5js.ProcCallLike#setStartingEnv+impls. Use in r5js.Continuable.
runtime+misc: remove r5js.Continuable#setStartingEnv.