Parametrize sorting networks on the swap function

[Morwenn]

Papers like Engineering Faster Sorters for Small Sets of Items by Bingmann & al. and Applying Sorting Networks to Synthesize Optimized Sorting Libraries by Codish & al. highlight the need for cpp-sort to be able to parametrize sorting networks on their swapping function.

This is actually a bit tricky considering that there's already iter_swap as a customization point which depends on the iterator type and swap on the value type. Even our own swap_if optimization in the library's internals is only applied when iter_swap isn't picked first. I guess that it would be possible to hijack that by wrapping some type somewhere in a dummy template to make sure that overload resolution picks swap(sorting_network_tag<T>&, sorting_network_tag<T>&); and that this overload calls the sorting function associated to the sorting network, but that's more templates, more random overload resolution and obscure customization points. It would also only work with stateless function types associated to the sorting network type, which is limited but should be enough for a compare-and-swap function.