Decompositions of CnX gates with ancilla qubits

[jpacold]

Description

Implements CnX decompositions from Maslov (2015). Related to #1725.

Opening this as a draft because there is a problem with the new test that I don't understand, and I wanted to check if it is reproducible in CI. The test passes if I run it with ./test-tket "Scenario: Test CnX_vchain_decomp". If I run the whole suite with ./test-tket, it fails when checking the unitary matrix of the synthesized circuit. I'm still not sure why this is happening; the same check seems to be OK on the Python side.

The paper includes two decompositions (given in Proposition 4 and Proposition 5), both using $\lfloor (n-1)/2\rfloor$ ancilla qubits to implement a gate with $n$ controls. Qiskit seems to have a simplified version of the one described in Proposition 4, using roughly the same gate counts but more ancillas ($n-2$).

[cqc-alec]

Strange, it seems it only fails on MacOS, and there is a mysterious "bus error" appearing. This reminds me of an issue @sjdilkes encountered recently, which I'm not sure he got to the bottom of...

[jpacold]

Still trying to understand the problem. For now I have moved the relevant part of the test to Python.

[yao-cqc]

Thanks! Everything looks good to me

[cqc-alec]

Thanks @jpacold ! Just need to update from main and probably bump the tket version again, and then we can merge this.