Speedup syntactic_implication() by using a cache.
* src/ltlvisit/syntimpl.hh (syntactic_implication, syntactic_implication_neg): Move as member of ... (syntactic_implication_cache): ... this new class, that holds a cache of results to speedup these functions. * src/ltlvisit/syntimpl.cc: Adjust to use (lookup, populate, and cleanup) the cache. * src/ltltest/syntimpl.cc: Likewise. * src/ltlvisit/reduce.hh (reduce): Take an optional syntactic_implication_cache parameter. * src/ltlvisit/reduce.cc: Adjust to use a syntactic_implication_cache. * src/ltltest/equals.cc: Call dump_instances() to help debugging.