Prefix char{16,32}_t with abc namespace in character literal macros

Also clarify that when non-native, char{16,32}_t are in abc::.
Include abc/core.hxx before checking for ABC_CXX_PRAGMA_ONCE
Explicitly cast to return type to suppress warnings
Suppress signed/unsigned mismatch compiler warnings
Suppress unnecessary MSC16 warnings
Suppress warnings from MSC16 system headers