Reorganize header files to avoid cross-header forward declarations
Make abc/str_iostream.hxx part of abc/core.hxx
Remove incorrect header file inclusion
Rename include/abc/*to_str_backend.hxx to *to_str.hxx
Only specialize abc::to_str_backend for char*_t only if they’re native types
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
Remove support for non-host-encoded abc::*str classes
Merge abc::_raw_str into abc::str_base

Now possible, since the latter is no longer a template.
Remove support for non-host-encoded abc::char_range
Remove now-unnecessary this-> qualifier
Add missing ABC_TRACE_FN
Update documentation