implement simple quantifier heuristics

The quantifier heuristics library is powerful, but often slow. The
unwind conversions are fast, but have serious restrictions. This commits
adds simple quantifier heuristics. They are based on the techniques
developed for quantifier heuristics library, but only search gap guesses
that don't contain free variables. Some very limited experiments suggest
that it is similarly fast as unwind. It can handle all the cases unwind
can handle. However, it supports also a much richer syntax. Moreover, it
can instantiate in addition to universal and existential quantifiers
also unique existential quantifiers, eliminate select and some.
Moreover, it knows about pairs and lists.