bin: release all subformulas between runs
Fixes #262, reported by Maximilien Colange. * bin/common_output.cc, bin/common_aoutput.cc, bin/common_aoutput.hh: Clear the set of atomic propositions if --stats=%[...]x was used. * spot/twa/bdddict.cc: Release any formula associated to a BDD when it is unregistered, do not wait for the dictionary's destruction. This was the main culprit for #262. * tests/core/ltl2tgba.test: Add test cases. * NEWS: Mention the bug.