Modernizing the C++ API

[zwimer]

Hi, I'm currently developing a native version of claripy in C++, and thus am using Z3's C++ API.

The C++ API seems to have been written in C++03 or before; at least functionally. It seems to be missing a fair number of core C++ features from C++11 and higher. One example is move semantics (I made a PR for this earlier); another is noexcept.

That said, there is a good chance I may contribute to do just this. If I do, what considerations should I be mindful of? Perhaps we don't want to go to C++20 but rather C++14, for example.

In general I'd like input about things that would be welcomed and things that would not if I were to make large changes to improve the API a bit. Guidelines about what might make any PR I make moot as it wouldn't be merged, if you will.