ast: r5js.ast.Quasiquote#processQuasiquote: make a branch unconditional.
ast: r5js.ast.Quasiquote#processQuasiquote: remove a type suppression.
ast+runtime: r5js.ast.Quasiquote#processQuasiquote should return ProcCallLike.
ast+runtime: r5js.ContinuableHelper#toContinuable should return ProcCallLike.
runtime: remove r5js.Continuable.
parse+runtime: r5js.Assignment should create its own Continuation.
parse+runtime: r5js.newProcCall: create a new Continuation where possible.
runtime: remove r5js.Continuation#getAdjacentProcCall.
parse+runtime: remove r5js.newBranch() in favor of new r5js.Branch().
runtime: inline Continuation construction into r5js.newProcCall where poss.
parse+runtime: remove r5js.newProcCall() in favor of new r5js.ProcCall().
runtime: remove unnecessary Continuation construction from Assignment ctor.
runtime+misc: remove r5js.newIdShim() in favor of new r5js.IdShim().
parse+runtime: move r5js.Continuation#desugarMacroBlock into ParserImpl.
runtime: factor out dynamic-wind-related Continuation behavior into subclass.
runtime: factor a helper method out of r5js.Continuation#evalAndAdvance.