I would like to support iterator feature by adding a few member functions, `begin()`, `end()`, `rbegin()`, `rend()` and their corresponding constant version. I think it's necessary for dynamic_bitset to have this feature through my thinking and other developer's opinions ( https://m.weibo.cn/5027267375/4757345970424490). Now I have submitted a pull request for the feature with code, docs and test case. Please review my code at your convenience. Thanks!