Support added for exceptions with ASYNCIFY

The general scheme is not very complicated. Changes:

1. Rather than doing `dynCall_vi` on the *foo*__asnc_cb function, we use `invoke_vi`. This is essential if the callback functions are to throw exceptions.
1. If a callback did throw an exception, then we should reenter the parent callback function only if it checks for exceptions at the resume point. Otherwise, the exception should propagate up. We check for this using an extra boolean in the async context: `emscripten_alloc_async_context` now has an extra argument which the C++ code uses to indicate whether the async function is called with "invoke" or "call". (So the `invoke` argument to `emscripten_alloc_async_context` is set when `__THREW__` is checked immediately after the async call.)
1. If the exception isn't caught anywhere, we rethrow it finally so that an error message is provided (admittedly not a very clear one, but it's no worse than when an exception is throw out of a synchronous `main`).

This pull request works in conjunction with the pull request to the fastcomp compiler.
Yup, I saw what you meant after looking at the comment more closely! I've expanded the comment a bit, and discovered a little bug while thinking it through:

If an __aysnc_cb function uses alloca (or anything that calls stackAlloc) then uses realloc_async_context to call another async function, then there's a problem! realloc_async_context tramples on the stack. I've added an assertion to catch this case at least; we should fail clearly rather than corrupting memory an application is using.