This introduces a general way to use public enums both in the C++ and
the (upcoming) C API. For the C++ case, we now use enum classes rather
than enums for public enums. This also includes definitions for
C API to_string conversions for public enums. C API definitions in
cvc5_types.h are only included from the C API, guarded via a macro
(thus, for now, not included yet when the header is included).