Port: synchronous operations with SharedArrayBuffer

[james-pre]

@jvilk:

If usable SharedArrayBuffers are detected when setting up the connection to/from the worker, use them for synchronous API calls.

@fallenoak:

@jvilk Out of curiosity, would this permit an async FS being used as a sync FS for Emscripten? I'm assuming no, but maybe I haven't quite wrapped my head around something.

@jvilk:

@fallenoak Yes, it would, but only if your emscripten code is running in a WebWorker.

jvilk/BrowserFS#162