runtime+misc: r5js.Continuation.nextContinuable_ should be a ProcCallLike.
runtime: remove apparently dead code from r5js.Continuation#evalAndAdvance.
runtime: make an overridden method protected, not private.
parse+runtime: simplify r5js.newAssignment.
runtime+misc: r5js.trampoline should take a ProcCallLike, not Continuable.
runtime+misc: ContinuableHelper#appendContinuable should use ProcCallLike.
runtime+ast: rename "appendContinuable" methods => appendProcCallLike.
ast+runtime: strengthen two return type annotations.
parse+runtime: simplify r5js.newBranch.
parse+runtime: r5js.newBranch should take ProcCalls, not Continuables.
misc: r5js.Datum#desugar should return ProcCallLikes, not Continuables.
misc: simplify r5js.newIdShim.
parse+runtime: simplify r5js.newTopLevelSyntaxAssignment.
parse+runtime: simplify r5js.newTopLevelAssignment.
parse+runtime: r5js.Continuation.desugarMacroBlock should return ProcCallLike.
parse+runtime: simplify r5js.newProcCall.