Async support

[lovelydinosaur]

Database validation needs an async/await interface. ValidationResult should be an awaitable, and should error if __bool__ or __iter__ are called, and it has awaitable validators, and has not yet been awaited.

[lovelydinosaur]

We'll keep the async/await parts optional, and seperate from the main value validation, so that it's super clear inside validation if and where database access might occur.