dynamic_bitset<> find_next() improvements

This pull request should provide some performance improvements to dynamic_bitset<>::find_next() by simplifying bit operations and using built-in instructions for finding the lowest bit. It also provides dynamic_bitset<> hash implementation (so it could be used by unordered_map/set<>).

There were corresponding tickets in boost.org trac for years already:
* https://svn.boost.org/trac/boost/ticket/2841
* https://svn.boost.org/trac/boost/ticket/5158
* https://svn.boost.org/trac/boost/ticket/5159

But there were no response from the maintainers. Also my post to boost.org mailing list had seen no feedback.
So this is my attempt to get some feedback via the new Github-based boost infrastructure.