Make SelectSwapQROM more idiomatic

[mpharrigan]

The figure from the paper looks like this

the current decomposition looks like

Some more intermediate-abstraction bloqs could clean this up. For example: the list of CNOTs could be wrapped in a bloq. The decomposition currently uses the cirq interop which does a lot of splitting and joining; as well as allocating. Perhaps more thoughtfully routing the registers with e.g. Partition could clean things up.