[tstr] avoid tstr_advance() in drivers get_constants functions
[tstr] avoid tstr_advance() in 'rank' calculation
[tstr] avoid tstr_advance() tstr_to_upper_lower()
[tstr] make tstr_find() accept a 'const' tstr
[tstr] make tstr_piece() accept const tstr
[fs] avoid copying file names when parsing file paths (also avoids tstr_cut())
[tstr] remove tstr_cut() - not used anymore