feat+refactor: Bump MSRV to 1.83.0 and maximize const

[Daniel-Aaron-Bloom]

Find every fn which can be const (as of 1.83.0) and make it so. This involved moving all the implementations out of the ArrayVecImpl trait, which eventually left it unused, so I removed it.

You could probably also remove MakeMaybeUninit and just have new_const call new if you wanted to.

[tbu-]

What's your motivation for this pull request? This currently raises the MSRV quite substantially, so it should have a good motivation, at the very least.

[Daniel-Aaron-Bloom]

I made this crate, and doing that was my motivation.

The summary is: now that &mut is const-compatible, you can make almost everything in ArrayVec into a const fn. Having a mut ArrayVec at compile-time is extremely powerful.

For now I've just forked this crate as arrayvec-const, but that's basically just a hack.

[Daniel-Aaron-Bloom]

Do you think this would be better served via a feature-flag?