Yangoon core.udpsocket

This adds a provider for core.udpsocket:
UWNetworksLab/freedom#52

Based quite heavily on chrome-fsocket.ts.

Also some bits and bobs of plumbing which aren't strictly necessary right now but will be useful for the next merge, which will consist of a "UDP server" (basically, a counterpart of tcp.ts).

This works pretty well with some toy code I have which adds UDP support to the proxy (pretty far from ready for review).

There's also a test. It's far from bulletproof but it does check that we're making the right calls to chrome.sockets.*.