Support `async fn` in trait methods (async/await follow-up)

As noted by the author of the async/await RFC during its discussion:

If the answer is just "this can't be used on trait methods", that removes a lot of the utility of this feature, especially for libraries.

This RFC does not propose enabling async trait methods. I agree that trait methods are an extremely valuable use case for async notation, and from my perspective it is the absolute highest priority extension after this initial MVP proposal (in contrast to things like async generators).

https://github.com/rust-lang/rfcs/pull/2394#issuecomment-379704976

Now that the async/await MVP is tentatively approaching stabilization, this is the next logical feature that users are going to start asking about. Discussion of how to support this feature is currently fragmented across various other async/await issues threads, so I have opened this issue in order to centralize discussion (and hopefully to have someone in-the-know summarize the current obstacles and plans).

cc @withoutboats

Start by reading RFC 2394 and its linked discussion, then review the other async/await issue threads referenced in this report. The issue is complete when the obstacles to async fn in trait methods and an agreed implementation plan are documented clearly enough to guide follow-up work.