Async trait FFI

[lisasgoh]

hey, does this currently work for async traits/if not, are there any plans to support it?

[h33p]

There are plans to support it, and I have an idea how to do this quite efficiently, but it will take a while to materialize.

Essentially, I need to hide RetTmp objects on the other side of the allocation. After rust-lang/rust#138424, I'll then be able to store resulting futures there without incurring allocation cost.