Allow quilc to find the shortest gate sequence possible

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It Would Be Nice (TM) if quilc could be used purely as an algebraic simplifier (namely for the QVM!) to reduce the number of gates total in the program. I don't necessarily mean fusion, but a re-expression of the program to be simpler.

[jmbr]

Exploiting knowledge of commutators and some basic simplifications on top of that might already be enough to make an impact.