Supporting async functions

This is only an early version. 
My hand-written tests as well as some PDCurses demos are working, but there are still some not working.

This PR is mainly for code review, because I think that the approach has been determined.

A library is necessary to support manipulating async stack frames. asm.js is not supported yet, but it won't be difficult, as there are only number operations (no references to functions)

kripken/emscripten#2482

Function pointers are not supported yet.


would be better to do `lower-emscripten-async-functions` (then commandline would be `--lower-emscripten-async-functions`