add array::FillError similar to array::IntoIter

[lcnr]

Taken from #69985 without actually implementing FromIterator so we can merge this without having to worry about
stabilizing anything.

While implementing new methods for arrays, this is very often needed and we otherwise need to reimplement this in each new method.

A huge thanks to @lperlaki, who has done most of the work necessary for this PR as part of #69985.

To summarize, this PR adds struct array::FillError<T, const N: usize>, which is intended as part of the return type of FromIterator for arrays, once we implement this (requires at least stable min_const_generics). The current concept is to implement FromIterator for Result<[T; N], FillError<T, N>>, returning a FillError if the iterator had less than N elements.

Similar to IntoIter, we don't yet actually implement FromIterator but instead add a probably permanently unstable new method to FillError, meaning that one can now write FillError::new().fill(iter).unwrap() to collect into an array, which was previously not cleanly possible.