Migrate express functionality and AbstractRepresentation type to QuantumInterface

[apkille]

#92 qojulia/QuantumInterface.jl#46
The problem with this PR and the one in QuantumInterface is that a type piracy is introduced here:

express(s, repr::AbstractRepresentation) = express(s, repr, UseAsState())

I suppose we could migrate AbstractUse and its subtypes to QuantumInterface as well, but

function express(state::Symbolic, repr::AbstractRepresentation, use::AbstractUse)

would then be a type piracy. Plus AbstractUse is intended for dealing with edge cases in symbolic-to-Clifford translations, so to me it doesn't make sense to migrate it up to QuantumInterface.

[apkille]

@Krastanov

[Krastanov]

Looks good! Three minor things:

• The comment about AbstractUse -- I think it makes sense to move this too. E.g. if QuantumClifford is converting an Operator it needs to know whether the user wants a Stabilizer or a CliffordOperator
• The docstring update (or move)
• Check that express still shows in the documentation -- now that it does not belong to QuantumSymbolics, it might need to be explicitly included.

Otherwise feel free to merge once CI passes, the Project.toml is bumped, without waiting for another review. I guess that requires the other PR in QuantumInterface to be merged first.

[Krastanov]

Marking it as a draft to remove it from my review queue for now. Convert it back when ready and merge (or ping me to merge).

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 75.22%. Comparing base (be8cb24) to head (36f3903).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #93      +/-   ##
==========================================
- Coverage   75.46%   75.22%   -0.25%     
==========================================
  Files          19       19              
  Lines         803      791      -12     
==========================================
- Hits          606      595      -11     
+ Misses        197      196       -1     

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[apkille]

• Check that express still shows in the documentation -- now that it does not belong to QuantumSymbolics, it might need to be explicitly included.

It still shows in the docs.