chore: clippy nursery

[StuartHarris]

Fixes for clippy::nursery, including adding to CI

[ManevilleF]

Nice ! clippy::nursery is a gold mine of useful lints !

I would suggest adding the lints in the Cargo.toml directly and be able to selectively disable some, as lots in nursery (though less than in pedantic) can be false negatives.
I would also advise against using missing_const_for_fn or be particulary careful on which public functions become const, as rolling this back will be a breaking change