Add context comments for distant #if/#else directives
Add code documentation for abc::to_str_backend and related classes
Merge abc::_str_to_str_backend_base into abc::_str_to_str_backend

The mid-class template abc::_str_to_str_backend provides no functionality
over its non-template base abc::_str_to_str_backend_base, so combine them
into abc::_str_to_str_backend, which is now not a template anymore.