We're still missing a lowering impl for a cirq.CircuitOperation. We could just inline those, but depending on the config it may be that a circuit operation is e.g. a for loop (repetitions).

It's also possible to control an entire CircuitOperation by a qubit, which I'm not entirely sure how to lower yet.

This is a blocker for roundtrip support (squin -> cirq -> squin), as subroutines in squin are potentially mapped to a CircuitOperation (see #311 ).