ability to "bundle" more errors before the first suspend point in an async function

[andrewrk]

I'll expand on this later but I need an issue to link to.

• When you await in the same stack frame that you async, there does not need to be a coroutine allocation.
• When you async before the first suspend point in a coroutine frame, this can be made to not fail, because the allocation could have been bundled up with the callee's coroutine frame

[andrewrk]

I created #1260 for the first bullet point. I'll let this issue become the second bullet point.

[andrewrk]

Obsoleted with the merge of #3033. async can no longer fail.