- remove static variable 'string_count'
- improve and document methods debug() and hexdump()
This commit does not resolve issue 601 but removes some development
leftovers.
Methods debug() and hexdump() left for debugging as requested.
The previous name existed already with different case (fl_string.h)
in the FL folder which broke the build on macOS and Windows.
This may be a temporary fix - until I find a better way.