[dynamic_bitset] Add iterator feature

From: GaoZIYU (gaozy0825_at_[hidden])
Date: 2022-09-11 14:56:30

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!