(refactor): remove ArraySubset unchecked methods

[ilan-gold]

Towards #52, making the API surface of ArraySubset will help determine what common operations can be moved to a trait

The two commit messages (so far) should signal what is being removed (so far), although they might have a non-trivial dragnet i.e., removing one function entailed removing others.

I haven't benchmarked this yet, but the size of the code-diff smells positive. Hopefully there's no performance knoc

While going through this PR, though, I learned about https://doc.rust-lang.org/reference/items/generics.html - we could maybe remove the dimensionality checks altogether if we parametrize the dimension? I'm not sure about this one though, would be curious to learn more

[LDeakin]

Yeah, all of these unchecked variants probably make negligible difference to perf. I'll do a benchmark when your PR is ready to go.

While going through this PR, though, I learned about https://doc.rust-lang.org/reference/items/generics.html - we could maybe remove the dimensionality checks altogether if we parametrize the dimension? I'm not sure about this one though, would be curious to learn more

Definitely don't want to parameterize on the dimensionality, as that would cause massive code bloat to monomorphisation.