Reset simplify and expression hashmaps when clearing.

Reset simplify and expression hashmaps when clearing.

If a hashmap got very big, some methods like clear would take a long time (using the hasmap that is linked to on my machine). The clear()
function traverses each of the buckets in the hashmap seeing whether it had children. If the hashmap previously had millions of elements, even if it
now contains just a single element, then there would still be millions of buckets. The clear() function would take much longer than it needed to.

These changes were inspired by testcase15.stp.smt. On my machine, this patch speeds up STP on that benchmark by 20 times.

git-svn-id: https://stp-fast-prover.svn.sourceforge.net/svnroot/stp-fast-prover/trunk/stp@67 e59a4935-1847-0410-ae03-e826735625c1