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.