[YAGGLER/TOOLS/BAD_TYPE]: added a 'bad_type' class (used in tuple, ...) to specify 'this type is a bad type'.
[YAGGLER/TOOLS/TUPLE]: empty tuples are now valids
[YAGGLER/TOOLS/TYPE_LIST]: removed use of tuples where a simple ct_type_list would be ok.
[YAGGLER/TOOLS/TUPLE]: modified code to take advantage of empty tuples