runtime: add r5js.ProcedureLike interface.
runtime: r5js.procspec.PrimitiveProcedure_ should implement r5js.ProcedureLike.
runtime: r5js.ProcCall#evalAndAdvance: wire in r5js.ProcedureLike support.
runtime: make r5js.procspec.PrimitiveProcedure_ a base class.
runtime: r5js.procspec.PrimitiveProcedure_ subclasses should be ProcedureLike.
runtime: make r5js.PrimitiveProcedure mostly useless.
runtime+misc: remove r5js.PrimitiveProcedure interface.
runtime: make r5js.procspec.PrimitiveProcedure_#Call private.
runtime: r5js.Procedure should implement r5js.ProcedureLike.
runtime: (x instanceof r5js.Procedure) => rj5s.ProcedureLike.isImplementedBy(x).
runtime: r5js.Continuation should implement r5js.ProcedureLike.
runtime: remove circular dependency in datum_util.js.