Support for PumpMessageLoop, needed for webassembly

[fizx]

Depends on (extends) #356's []byte support.

I haven't figured out a nice way to polyfill this into WebAssembly.compile yet, but this at least allows you to PumpMessageLoop at all. WebAssembly.compile uses both background and foreground tasks, and if you don't pump the message loop, the foreground tasks never execute, leading to a hanging process.

See the test case for how to do this. I'm following the idea of make it possible first, then make it nice later.