Version 0.21

• make \__xsim_set_file_signature:nnn public
• make \l__xsim_file_path_and_name_tl, \l__xsim_file_name_tl and \l__xsim_file_path_tl public
• introduce \XSIMsetfilebeginX and \XSIMsetfileendX (xsimverb)
• reduce comment line in automated comments in exercise and solution files
• add option <tag>/use-unmatched
• document option blank/fill
• add counter foreach type and collection that holds the number of exercises of a type in a collection
• fix issue #90
• correct file hooks (cf. issue #98)
• implement issue #88
• fix issue #96
• use! and print! had gone lost; add them back in.
• add new options match-all/match-any for the collection conditions; resolves issue #94.
• new \ForEachUsedExerciseByOrder, used in the table templates; resolves issue #85