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.
Remove non-prominent features from README Introduction to shorten the list
Add Compatibility section to README
Minor style fixes
Remove pedantic Coding and formatting style section from README
Add Why/When section to README
Fix sentence
Improve code documentation
Add “How to use it” section in README
Add basic documentation on app modules
Add documentation for a few more methods
Add signature documentation for functions in abc::atomic.
Fix code documentation
Document signature of functions in abc::byteorder.
Rename STATIC_BYTEORDER_*() macros to ABC_BYTEORDER_*()
Comment remaining functions in bitmanip.hxx
Complete code documentation for char.hxx
Fix incorrect return values for abc::char_range_::cr{begin,end}()