<regex>: Small cleanups

[frederick-vs-ja]

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.

[StephanTLavavej]

Thanks! 😸 FYI @CaseyCarter I pushed minor changes after you approved.

Also, in case it wasn't just a typo - note that "simply" is an adverb, while "simplify" is a verb.

[StephanTLavavej]

I'm mirroring this to the MSVC-internal repo - please notify me if any further changes are pushed.

[StephanTLavavej]

I had to push an additional commit to fix regressions found by our Perennial test suite. See the cited Standardese for const-default-constructible and https://godbolt.org/z/GeWYxf37f .

[StephanTLavavej]

Thanks for cleaning up one of our dustiest headers! 🌬️ 🧹 🪄