cpp interface improvements

[samuelpmish]

working on some fixes and improvements for the new cpp interface:

• fixing inconsistent size_t vs std::size_t usage
• use automatically generated ctors for enzyme::Active, enzyme::Const, ... , to support brace-initialization
• add CTAD guides for C++17 and later (so users don't have to explicitly name the argument types)
• added a new directory in tests for c++ interface examples and unit tests with CTest support (WIP)
• disambiguate forward_by_reference implementation (see New C++ interface with lambda #1785)

I think @wsmoses also mentioned something about casting the function pointers to the appropriate types-- can you clarify what you want to happen and I'll add a test for it.

I'm also seeing an error in the return type deduction for Forward mode, so I'll look into that.

[wsmoses]

Can we move the sugar tests from fwd and reverse mode integration folders here too?