Memoize unification to speed up URE

[ngeiswei]

One source of computational overhead in the URE is the unification between rule conclusions and targets when going backward, or rule premises and sources when going forward.

Often the same unification operation is carried over. Add to that, unification is stateless. Thus is a perfect candidate for memoization.

[ngeiswei]

Some more precisions, the calls to be memoized are
https://github.com/opencog/atomspace/blob/master/opencog/rule-engine/backwardchainer/ControlPolicy.cc#L119
for the backward chainer, and
https://github.com/opencog/atomspace/blob/master/opencog/rule-engine/forwardchainer/ForwardChainer.cc#L313
for the forward chainer.