Remove and cleanup functions in std::vec

Visit the free functions of std::vec and reimplement or remove some. Most prominently, remove `each_permutation` and replace with two iterators, ElementSwaps and Permutations.

Replace unzip, unzip_slice with an updated `unzip` that works with an iterator argument.

Replace each_permutation with a Permutation iterator. The new permutation iterator is more efficient since it uses an algorithm that produces permutations in an order where each is only one element swap apart, including swapping back to the original state with one swap at the end.

Unify the seldomly used functions `build`, `build_sized`, `build_sized_opt` into just one function `build`.

Remove `equal_sizes`