Add `ToConstraintFieldGadget` bounds to `CurveVar` and `FieldVar`

[winderica]

Description

This PR adds the requirement that structs that implement CurveVar or FieldVar should also implement ToConstraintFieldGadget, which could be convenient e.g. when we want to extract the underlying FpVars from a generic CurveVar without specifying + ToConstraintFieldGadget<...> every time.

Since both implementations of CurveVar (i.e., SW ProjectiveVar and TE AffineVar) and all implementations of FieldVar (i.e., FpVar, QuadExtVar, CubicExtVar, and EmulatedFpVar) already implement ToConstraintFieldGadget as well, this PR does not introduce any change in the behavior of existing code.

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[Pratyush]

I think a semantically cleaner version of this would be to add x() and y() operations to the CurveVar trait.

[winderica]

@Pratyush Yeah that makes sense! Will add these methods, as well as xy(), to CurveVar.

But I still think adding ToConstraintFieldGadget to CurveVar is beneficial. x() and y() are called more likely for exposing the details of the point, while a developer may invoke to_constraint_field() without caring about the implementation details (e.g., whether the result contains infinity, which is true for SW points but not for TE points). What do you think?