Add CnX decompositions with ancilla qubits

[sjdilkes]

See qiskit implementations here -> https://docs.quantum.ibm.com/api/qiskit/qiskit.circuit.QuantumCircuit#mcx

https://arxiv.org/pdf/1508.03273

[jpacold]

Could I take part of this, say implementing MCXVChain?

If so, should it just be a new function in CircPool.hpp?

[yao-cqc]

@jpacold Much appreciated, thanks for taking this on!

Implementing MCXVChain in CircPool and binding it in circuit_library.cpp would be a good start.

In the bigger picture, tket doesn't have a mechanism to manage ancilla qubits, so integrating this CnX decomposition method into the current rebase pipeline wouldn’t be straightforward. It might be useful to support CnX gates as Boxes, allowing users to specify how each should be decomposed. But for now, I think adding MCXVChain to CircPool is a necessary step forward.