Allow '_mixture_' to return gates instead of just raw unitary matrices

[daxfohl]

Fixes #1657

Updates everywhere _mixture_ is called, to ensure gates can be handled. Changes BitFlip to be a mixture of X and I gates as an example.

Also updates apply_mixture to take advantage of this. Currently apply_mixture calls mixture() and passes the results to apply_unitary one-by-one. But since mixture() converts the gates now returned by _mixture_ to unitaries, that prevents it from being able to use the more efficient _apply_unitary_ strategies that the gate itself may provide. This PR changes apply_mixture to use the values returned by _mixture_ directly instead, allowing for the optimizations of those gates have them.

Some of the code could be simplified if we did #7039 (plus allowing unitary(val: <raw np unitary>) => val), but that's purely aesthetics. (Side note: the three different protocols are each implemented with slightly different styles; it might be good to standardize on one).

Note: since the mixture protocol now converts non-numpy return values to unitaries, the existing unit tests that used strings 'a' and 'b' no longer work, and had to be changed to proper unitaries. That shouldn't be a problem in any real-world use.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.65%. Comparing base (4436b85) to head (08d9837).
Report is 39 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7048      +/-   ##
==========================================
- Coverage   98.66%   98.65%   -0.01%     
==========================================
  Files        1106     1106              
  Lines       95914    95869      -45     
==========================================
- Hits        94630    94581      -49     
- Misses       1284     1288       +4     

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[pavoljuhas]

Looks great, thank you!

Please see comments before merging. (Non essential feel free to skip them)

[daxfohl]

Labeling as BREAKING since unit tests had to change. However the unit test were artificial, returning strings from the mixture implementation. I can't think of any real-world thing this would break.