Introduce CPHASE-to-ISWAP translator

[ecpeterson]

There is a translator chain for parametric compilation of CPHASE gates to ISWAP architectures: CPHASE gets compiled to a pair of CNOTs, each CNOT gets compiled to a pair of ISWAPs. This is unnecessarily bulky: a CPHASE can be parametrically expressed using a pair of ISWAPs. Implement this decomposition as a translator and install it for ISWAP-native architectures.