feat(breaking): make FromIteratorFixed more generic

[Daniel-Aaron-Bloom]

Presently, it's not really possible to express FromIteratorFixed as a generic bound. For example

fn build_cs<C, const N: usize>(vs: Vec<[u64; N]>) -> Vec<C>
where: C: FromIteratorFixed<WHAT_TO_PUT_HERE, N> {
  vs.into_iter().map(|v| v.into_iter_fixed().map(|i| i+10).collect()).collect()
}

This is because the FromIteratorFixed currently locks in the iterator type at the trait level, while all the IteratorFixed functions like map completely hide the type behind impl Iterator.

The solution is to move the iterator type down to the from_iter_fixed method, only preserving the Item type at the trait level. This is also logically correct as the specifics of the iterator type should not restrict implementations of FromIteratorFixed.

This is technically a breaking change, but I don't think it's likely to impact any casual users of this crate, only downstream crates which do things like add implementations of FromIteratorFixed will have to perform minor updates to signatures.

[usbalbin]

Thank you! :) 🎉