A function at risk of "naive misuse", it is only accessible via the C API.

This commit adds basic support for it using the same mechanism we use for
signals.  We also have the GIL-check mechanism, but that would be a bit more
work to get working right now due to the fact that our GIL-checks don't support
throwing exceptions.  Doing the async-exc check during signal checking means
that we will throw the async exc faster than CPython does.  It also means
that there are some pathological cases where with a lot of threads and a lot
of async excs we will probably have much worse performance.  But as long as they
are rare I think this commit shouldn't add any steady-state performance costs.