Support C++20 with string literal support.

While the current syntax is very clear, it constrains the usage to only clang and is not portable. It also has secondary issues, e.g. implementation cannot be split from the header when using a JNI Bind type because the compiler cannot have a const auto& non type template parameter with an out of line definition.

Furthermore, the use of concepts and non-type string literals should allow for simplification and also begin to open the project up to adaptations for protos (and other APIs).