Deprecate LinearFunctionsSynthesis transpiler pass

[alexanderivrii]

Summary

We already have a high-level-synthesis transpiler pass that is able to synthesize linear functions using the plugin interface. The original LinearFunctionsSynthesis transpiler pass is no longer needed, and we have decided to deprecate it.

In addition, LinearFunctionsSynthesis was previously based on the definition method provided in LinearFunction (which calls the specific cnot_synth algorithm). This may lead to confusion, as it does not use the synthesis plugin interface specified underlinear_function.default in entry_points.

This PR starts the deprecation clock on LinearFunctionsSynthesis, and temporarily reimplements as a special case of HighLevelSynthesis (using the synthesis method specified underlinear_function.default in entry_points).

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Additionally comments:

• The LinearFunctionsSynthesis is not used in the transpilation flow, as transpile calls HighLevelSynthesis and not LinearFunctionsSynthesis.

• Currently LinearFunction inherits from Gate (which eventually inherits from Operation). We have decided that it's fine to keep it this way.

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[coveralls]

Pull Request Test Coverage Report for Build 3943107070

• 8 of 8 (100.0%) changed or added relevant lines in 1 file are covered.
• 5 unchanged lines in 2 files lost coverage.
• Overall coverage increased (+0.09%) to 84.859%

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Files with Coverage Reduction	New Missed Lines	%
src/sabre_swap/mod.rs	2	99.54%
src/vf2_layout.rs	3	94.74%

Totals
Change from base Build 3942236674:	0.09%
Covered Lines:	65800
Relevant Lines:	77540

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💛 - Coveralls

[mtreinish]

We should maintain some tests of the old pass to ensure it works as is during the deprecation period.

[alexanderivrii]

I have added a test that the old functionality still works and a bug fix (shame on me, I did break the functionality), see 0309abe. I believe that a single test suffices, since most of the tests actually check the orhogonal block collection aspect of resynthesizing linear functions.

There was also a suggestion to extend the @combine(do_commutative_analysis=[False, True]) to something like@combine(do_commutative_analysis=[False, True], synthesis_pass=[HighLevelSynthesis, LinearFunctionsSynthesis]), however I did not end up using this since in one case we want to call

synthesized_circuit = PassManager(synthesis_pass()).run(optimized_circuit)

and in the other case we want to call

with self.assertWarns(DeprecationWarning):
    synthesized_circuit = PassManager(synthesis_pass()).run(optimized_circuit)

and I did not think of a clever way to nicely combine the two.

[mtreinish]

LGTM, thanks for adding the test