async functions have error return traces where appropriate

however the traces are not merged on `await` or async function calls
yet.

When an async function has an error set or error union as its return
type, it has a `StackTrace` before the args in the frame, so that it is
accessible from `anyframe->T` awaiters. However when it does not have an
errorable return type, but it does call or await an errorable, it has a
stack trace just before the locals. This way when doing an `@asyncCall`
on an async function pointer, it can populate the args (which are after
the `StackTrace`) because it knows the offset of the args based only on
the return type.

This sort of matches normal functions, where a stack trace pointer could
be supplied by a parameter, or it could be supplied by the stack of the
function, depending on whether the function itself is errorable.

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