Make abc/trace.hxx part of abc/core.hxx
Ensure string pointers are printed as strings
Add abc::to_str() support for more pointer types
Rename to_str_backend.cxx files to to_str.cxx for consistency
Rename to_str_backend test cases to to_str for consistency
Ensure string pointer is printed as string

Missed in 2ac0fdf07595e1313328eb408b2ede85ad61770f.
Remove support for printing char*_t pointers as strings
Fix _enum_to_str_backend_impl::write_impl()
Add test case for abc::to_str() of pointers