Async (de)serialization

[Rich-Harris]

No concrete API proposal yet, but we need a place to discuss the best way to handle async serialization/deserialization within devalue. This is most useful for promises, but it might also be good to be able to serialize streams/async iterables

[gr2m]

Would it make sense to start with just async serialization as a first step? The scope is small, it's immediately useful, and it doesn't require any changes to the wire format or deserialization.

Proposed API: https://github.com/gr2m/devalue/blob/async-serialization/README.md#stringifyasync-and-parse