[RFC] Registerless circuits and circuit composition

[ajavadia]

We propose simplifications to Qiskit circuits:

1. Registers are completely optional. They can be added where it makes sense. The qubits/clbits are the fundamental data, not registers.
2. The above makes it easier to compose circuits with each other, so you no longer need to convert circuits to instructions to anonymize them before appending onto another circuit.
3. All of the above makes it easier to treat gates/composite gates/subcircuits/circuits as the same thing.