OnceCell::set is unimplemented

[lewisfm]

We should make it so that OnceCell::set has side effects on the OnceCell instead of doing absolutely nothing.

[zabackary]

For context:

https://github.com/vexide/vexide/blob/main/packages%2Fvexide-core%2Fsrc%2Fsync%2Fonce.rs#L141-L152

[ion098]

Doesn't OnceCell::set need to be async for this to work?

[tropicaaal]

Will be closed by #361, since we're switching to async-lock primitives.