Wormblossom Async Utils

A crate of utilities for working with async code, making a few simplifying assumptions:

• nothing is panic safe, we assume panics to abort rather than rewind, and
• everything is !Send, futures will be executed on a single-threaded runtime.

The "synchronisation primitives" in this crate are not about synchronisation across threads, but merely to let different parts of programs .await access to some data.