* 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.