The current implementation of `CircuitData.add_qubit` (ditto `clbit`)
has linear time complexity because it reconstructs the list on each
addition, while the `CircuitData.qubits` getter silently clones the
list on return.  This was intended to avoid inadvertant Python-space
modifications from getting the Rust components out of sync, but in
practice, this cost being hidden in a standard attribute access makes it
very easy to introduce accidental quadratic dependencies.

In this case, `QuantumCircuit(num_qubits)` and subsequent
`qc.append(..., qc.qubits)` calls were accidentally quadratic in
`num_qubits` due to repeated accesses of `QuantumCircuit.qubits`.