Custom async for cleaner code

[nic-hartley]

Currently everything needs to be manually written as a state machine, so we can have proper concurrency. But there's already tech in Rust for being able to write relatively normal code, while the compiler turns it into a state machine: async.

I don't want to use normal async, i.e. tokio or async-std, for two reasons:

• The executors are design to work for system IO, whereas I'm mostly gonna be waiting for events and (in-game) time passing, so there's a lot of unnecessary features.
• That also makes them very no_std/WASM-unfriendly, whereas -- since I don't need them for IO -- a custom async executor could be much nicer about it.

The downside, of course, is that it's a lot of work, but I think it'd fit nicely into the engine.

The biggest issue I can foresee is the risk of intermixing asyncs. I have no idea how big the risk will be of someone trying to pass Tokio executors to me, or my futures to Tokio -- and that's a problem! It could be a very big problem! That said, I assume it's solvable, from how there's more than one async executor and people don't seem to have issues with accidentally intermixing them.