Async versions of `yoke` functions

[Enduriel]

Is this something you would be interested in a PR for, or is there some other reason this hasn't been done yet?

They definitely won't be as popular to use as the sync versions, but I'm in a situation where I need a try_map_project_async and I might make a fork for just that but it would be nice if this was in the upstream crate.

[sffc]

@Manishearth wdyt?

[Manishearth]

Please provide more details about the shape of the APIs you want.

I would be very surprised if an async version of the Yoke APIs were actually sound. Yoke strongly relies on being able to control what code runs inside its critical sections.