Add hint::assert_unchecked(len <= CAP)

[barakugav]

First of all, love the library! use it all the time 😄

This PR add a std::hint::assert_unchecked(len <= CAP) call in ArrayVec::len().
We know this assumption, and telling the compiler about it let it generate better code and eliminate some bound checks.

It also include a small benchmark for ArrayVec::clone() that demonstrate the (massive) speed up.

# before
> cargo bench clone
running 2 tests
test clone_512 ... bench:         345 ns/iter (+/- 20) = 1484 MB/s
test clone_8   ... bench:           4 ns/iter (+/- 0) = 2000 MB/s

# after
> cargo bench clone
running 2 tests
test clone_512 ... bench:         205 ns/iter (+/- 15) = 2497 MB/s
test clone_8   ... bench:           2 ns/iter (+/- 0) = 4000 MB/s

Clone is my specific use case, but im sure other generated code is more optimized when the compiler knows this.

std::hint::assert_unchecked was stabilized in 1.81 (docs), so I added a few lines for detection in a build.rs, hope that acceptable.

Thanks for the support in advance ❤️