* `await @asyncCall` generates better code. See #3065
* `@asyncCall` works with a real `@Frame(func)` in addition to
a byte slice. Closes#3072
* `@asyncCall` allows passing `{}` (a void value) as the result
pointer, which uses the result location inside the frame.
Closes#3068
* support `await @asyncCall` on a non-async function. This is in
preparation for safe recursion (#1006).