Add support for async/await (pending custom executors)

[helje5]

It would be really nice to support async/await in Swift 5.5, but alas, it requires thread hopping towards SwiftNIO, which makes it kinda useless.

So let's wait until Swift supports custom executors (and potentially SwiftNIO 3), which would presumably allow integration w/ SwiftNIO.

Peer to Macro issue: Macro-swift/Macro#3