dgram (udp) catchup with runtime/pull/595

Should be ready, but does require coordinated release with <del>https://github.com/tessel/runtime/pull/595</del><ins>https://github.com/tessel/runtime/pull/622</ins>.
Bleh, sorry, I meant to imply the alternate arrangement of having separate input vars. If that's not convenient or poor design I won't press the idea.