Added commented code that lets us evaluate a pdspec/property directly rather than going through EvaluationManager.
Unfortunately, this does not work correctly, which is why it is commented.  First, it requires me to type-check the property, which I could normally do, although the API isn't very good.  But this is more difficult because of generic methods (e.g., T identity(T)), which should work but I couldn't get them to.  In addition, for free synthesis I would have to re-type-check the pdspec for each value with a different static type (where for non-free synthesis I could do it just once).  This would be slow and complicates the code.  So I'm leaving this commented.  But it would be nice to enable it eventually.
This also contains some minor cleanups and bugfixes, mostly in the ExpressionEvaluator.
Tiny cleanup.
Split the part that computes expression probabilities out of the expression sorter so I can use it by itself.
Minor bugfix.
Added initial version of stochastic search.
Improved stochastic search.