Adding @defgroup and @ingroup doxygen tags into all headers
in the common folder that contain doxygen blocks.
This improves the structure, readability, and findability
of information in the resulting output.
This commit targets purely structure and does not deal with
the content of the currently existing doxygen documentation.
Compilers optimise these back into shifts on architectures where
shifts of negative integers work the same as mul/div, so this
solves the UB without actually causing any performance issue.
