Wasm freestanding (i.e. browser) via extern loop

[mitchellh]

Real goal: I want libxev to "work" in the browser so that applications using the event loop can be ported more easily (I have a real world example personal project I want to make work).

The idea is to support a "WasmExtern" backend that defers the actual event loop to the host environment (i.e. the browser). The host environment will call back into the module environment in order to trigger a "tick" and have callbacks be called.

This will make the consumer API slightly different since we can't just block on loop.run(.until_done). But consumers can check something like if (xev.Loop.is_async) or something and work differently.