Fix avoidable exponential blowup in unifier.

Change the code to do what the previous code tried to do, which is to
match free variables only after matching the more complex terms. This
avoids iterating over all possible subsets of the RHS.

The unidirectional unifier now works on examples such as a+b*c =>
d+e*f. This required a couple of changes to the algorithm, the main
one being that we allow variables to match against subtrees of the
expression.

Make CompiledExpression + pickling work on both Python 2 and Python 3.

Changed to use __getstate__() and __setstate__() rather than
__getinitargs__(), which is Py2 only.