Simplify and modernize metaprogramming utils

[halleysfifthinc]

My ultimate goal when beginning this PR was to add developer hints in the form of static_asserts to catch missed SuperType specializations (as mentioned in my last PR). I didn't get that far in this PR, but it has some helpful changes anyways. I started by working to understand (and then simplifying if possible) the metaprogramming utilities related to wrapping parametric types (e.g. apply_combination, etc). As a result, the commits in this PR are not all related, but they are all atomic (with one exception), so I can split this up if/as much as you want (or drop the more style based commits).

• I found a several cases where I was able to use more modern C++ type-trait features to reduce verbosity and improve readability IMO (e.g. std::is_same_v<T,U> instead of std::is_same<T,U>::value, etc). (The README says C++17 is the minimum required, but the CMake configuration actually requires C++20.)
• RemoveDuplicates, CombineTypes, and DoApply and ForEachType were all simplified to avoid or reduce the amount of recursion by using fold expressions.
  • I don't think the simplification with fold expressions for DoApply and ForEachType in 2ecb670 works without the CombineTypes refactor.
• Missing constexpr added to two if branches conditioned on a type-trait
• Some dead code was removed

[barche]

Many thanks for this, it's a great improvement!