1. Simply overly generalized traits and algorithms, which are actually specific to std::char_traits and sub_match.
2. Add const to some operator(). A (non-static, non-explicit-parameter) operator() without const usually implies the status of the function can be modified, which is not the case for the touched functors.
3. Use = default; and default member initializers for some classes.
4. Strengthend exception specifications for some funcitons.

Maybe in a future PR: Moving some function bodies into class bodies. Separation doesn't seem necessary to me, but the change is a bit large.