Quantum Chess: Decomposition into CZ gates

[dstrain115]

Quantum chess curreently decomposes moves into circuits containing square root of iSWAP gates (ie. ISWAP ** 0.5). It would also be good to be able to decompose these operations into CZ-based gates to support future processors.

In particular, the methods in the following files need to be able to support outputting circuits using CZ:

https://github.com/quantumlib/ReCirq/blob/master/recirq/quantum_chess/quantum_moves.py
https://github.com/quantumlib/ReCirq/blob/master/recirq/quantum_chess/controlled_iswap.py

This would be a good project for a 20% contributor interested in learning about quantum computing.