Fix equality and pickling of DAGCircuit with stretches

[kevinhartman]

Summary

During testing, I've found two bugs with DAGCircuit's handling of stretches:

1. Stretch capture order should not matter in circuit equality #13998
2. DAGCircuit does not properly pickle stretches #13999

They are fixed here.

Details and comments

I've also reworked DAGCircuit's tracking of stretches to be the same as what we do for vars. Namely, there's now just an identifier_info field (which replaces var_info) which makes it easier to detect name collisions between all identifiers in tracking, and also should make it easier to support native handling of stretches in the future.

To make things a bit cleaner, I've moved Var to lib.rs and defined a macro impl_circuit_identifier to reduce boilerplate between our Qubit, Clbit, Var, and now Stretch identifier types.

I've also moved the logic of pickling identifier info tracking from __getstate__ and __setstate__ to FromPyObject and IntoPyObject implementations.

To-do

• It looks to me like we don't have any testing of pickling / unpickling a DAGCircuit. Can that possibly be true?

[qiskit-bot]

One or more of the following people are relevant to this code:

• @Qiskit/terra-core

[coveralls]

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Details

• 186 of 209 (89.0%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.05%) to 88.161%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
crates/circuit/src/lib.rs	13	17	76.47%
crates/circuit/src/dag_circuit.rs	171	190	90.0%

Totals
Change from base Build 13816498497:	0.05%
Covered Lines:	72700
Relevant Lines:	82463

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💛 - Coveralls

[mtreinish]

It looks to me like we don't have any testing of pickling / unpickling a DAGCircuit. Can that possibly be true?

If we have any we get it indirectly through tests that are using multiprocessing (like transpiling > 1 circuit) because pickle will be used for IPC or deep copy which also internally uses pickle. That being said see #13980 for some explicit tests I had to add for another pickle bug that's been around since 1.3 (when we ported DAGCircuit to rust).

[kevinhartman]

see #13980 for some explicit tests I had to add for another pickle bug that's been around since 1.3 (when we ported DAGCircuit to rust).

Great, thanks! I've added a few tests based on those here to validate that stretches are preserved after pickling and deepcopy.

I also tested vars locally, but I found that there's a bug there that predates 2.0 so those tests aren't added in this PR (we ought to address that separately):

• DAGCircuit with vars does not compare equal after pickling #14007