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