support coroutine

A few questions

- How about a variance of `emscripten_coroutine_create` that allows customized stack size?
- Do we need `emscriptens_coroutine_destroy`, for those coroutine we don't need anymore, but still running?
- Do we need to save and restore STACK_MAX?
- The code is not actually asm.js validated, not sure how to make _malloc and _free work.
need a typedef for the function pointer type here, see other examples in this file (might be able to reuse a typedef that already exists, maybe)