Implement `await` syntax

[nikomatsakis]

As of this writing, we have not settled the precise syntax we will use for await. However, we have decided that we want to decide -- so it makes sense to get started on implementation. This issue is focused purely on implementation issues; the question of what syntax to use is covered in #57640.

[cramertj]

I've finished this, pending changes to whatever final syntax is chosen.

[estebank]

CC #51751

[cramertj]

@estebank Yeah, it fixes #51751 and #51719.