[libc++] Implement std::boyer_moore{, _horspool}_searcher

This mostly copys the `<experimental/functional>` stuff and updates the code to current libc++ style.

Reviewed By: ldionne, #libc

Spies: nlopes, adamdebreceni, arichardson, libcxx-commits, mgorny

Differential Revision: https://reviews.llvm.org/D121074