perf: call reserve method in set and map casters

[Skylion007]

Description

• Our STL caster was smart enough to reserve space in the list_caster. However, std::unordered_map and std::unordered_set also have the ability to reserve space ahead of time. This PR updates our other casters to make use of this. Reserving the size of the container is even more important for these types than list_caster since resizes will also call all elements to be rehashed. I am rather surprised it took so long for someone to notice this issue.
• I wanted to separate out the messy enable_if_t condition into a has_reserve_method alias, but kept getting syntax errors.
• Tested by adding static_assert false to make sure the proper reserve_method overload is called when appropriate.

Suggested changelog entry:

* Reserve space in set and STL map casters if possible. This will prevent unnecessary rehashing / resizing by knowing the number of keys ahead of time for Python to C++ casting. This improvement will greatly speed up the casting of large unordered maps and sets.

[rwgk]

Very nice, I really like this version!