Swap to using std's OnceCell once stablized

[james7132]

OnceCell is set to be partially stablized in Rust 1.70. (rust-lang/rust#105587).

If it releases as a part of Rust 1.70 and it's acceptable to bump the MSRV of this crate to it, it may be possible to replace the once_cell dependency of this crate with the stdlib's implementation.

[Amanieu]

I don't think there is a strong motivation to do that, and there will be a strong pushback against increasing the MSRV of this crate.