introduce realizability_simplifier to share more of ltlsynt's code
- spot/tl/apcollect.hh, spot/tl/apcollect.cc (realizability_simplifier): New class, built from code existing in ltlsynt, so that other tools may use this too.
- bin/ltlsynt.cc: Use realizability_simplifier.
- spot/twaalgos/aiger.cc, spot/twaalgos/aiger.hh: Adjust to use realizability_simplifier instead of relabeling_map.
- NEWS: Mention the new class.