Fix State Fidelity cache key

[woodsp-ibm]

Summary

Fixes #90

Details and comments

Switches from id() to _circuit_key() where the latter is that which the reference primitives use. Using id() is problematic, as #90 notes, though _circuit_key is designated a private method, and while its the better choice functionally, it feels a bit off to be using what is designated a private method. Its been used/stable for a while though. A choice might be to copy the whole routine here - but perhaps better to see if this changes going forwards in the way some unique key/hash can be obtained for a circuit.

[coveralls]

Pull Request Test Coverage Report for Build 6691648625

• 3 of 3 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.002%) to 90.556%

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Totals
Change from base Build 6691594462:	0.002%
Covered Lines:	6453
Relevant Lines:	7126

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💛 - Coveralls

[ElePT]

Sorry for my late review, but the change LGTM. I think that using _circuit_key() is the most reasonable fix at the moment. It's not ideal to be using a private method, but I expect interfaces to change over the past months and I think it makes more sense to merge the fix as-is now, and revisit the design in the future, after the primitives interface is stabilized (if anything, I feel like the public circuit_key() method would fit better in primitives.utils than here).