Make `And` a leaf bloq

[mpharrigan]

Specific changes that are needed to do this part of #873 :

• Any unit test that relies on the "sigma" return value of call_graph will change. This is brittle to begin with. Need to port to QECGatesCost
• Cirq simulation needs the cirq-style decomposition unless we override _kraus_ with custom kraus operators.
• Shim over the _t_complexity_ values for the time being.

[mpharrigan]

note: the keep argument can be removed from the unit tests in permutation_test.py after this is done

[mpharrigan]

There is now a flag called FLAG_AND_AS_LEAF which -- when set to True -- will turn And into a leaf bloq. There is a method called to_clifford_t_circuit() which returns a cirq circuit.

I've kept the decompose_from_registers method to support cirq simulation, because it's not straightforward to override the _kraus_ method

[fdmalone]

@mpharrigan can you remind me what the annoying chemistry things were, going to start working on this

[mpharrigan]

I don't think "annoying" is the right word; but many of the chemistry tests define a reference toffoli count, multiply it by 4, and assert that it matches deduced T counts. This is usually done via the sigma return value of bloq.call_graph(). Changing the status of And changes the contents of sigma. The tests need to be re-worked to compare the toffoli counts directly via new sigma values or porting to QECGatesCost.

[fdmalone]

@mpharrigan is there something else I can do here?

[anurudhp]

@fdmalone are you working on this? If not I have some wip code that i'd written for my other PR. I'm happy to polish that and make a PR

[fdmalone]

Go for it, I was mainly focused on cleaning up the tech debt of the chemistry bloqs, let me know if you hit anything.

…

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[mpharrigan]

I'm not sure what else needs doing. If tests pass after setting / removing the FLAG_AND_AS_LEAF then it's probably pretty close to being done-done