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.