std::vec: Replace each_permutation with a new Permutations iterator

Introduce ElementSwaps and Permutations. ElementSwaps is an iterator
that for a given sequence length yields the element swaps needed
to visit each possible permutation of the sequence in turn.

We use an algorithm that generates a sequence such that each permutation
is only one swap apart.

    let mut v = [1, 2, 3];
    for perm in v.permutations_iter() {
        // yields 1 2 3 | 1 3 2 | 3 1 2 | 3 2 1 | 2 3 1 | 2 1 3
    }

The `.permutations_iter()` yields clones of the input vector for each
permutation.

If a copyless traversal is needed, it can be constructed with
`ElementSwaps`:

    for (a, b) in ElementSwaps::new(3) {
        // yields (2, 1), (1, 0), (2, 1) ...
        v.swap(a, b);
        // ..
    }

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