Replace `lazy_static` with `once_cell::Lazy`

[cloudhead]

The latter is becoming standardized.

[vincenzopalazzo]

Maybe we could also move the implementation to the experimental one without importing new dependencies?

I used the rustc version and it works pretty well and looks stable enough, but the API looks like different from the once_clell I need to make double-check with the rust lib team on the stabilization road map of this feature

https://github.com/vincenzopalazzo/rio/blob/main/rt/src/runitime.rs#L79

[cloudhead]

How would that work? Doesn't it require nightly?

[vincenzopalazzo]

Yes for now yes https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021