refactor: frontend.Variable is now an interface, simplifies witness assignment and constant usage

[gbotrel]

This PR addresses few feedbacks.

It unfortunately introduces some breaking changes in the API.

• circuit.Define(curveID, api) is now circuit.Define(api)
• frontend.Variable is now an interface. Consequences to that is;
• api.Constant(x) is removed, doing myVariable = 2 instead of myVariable = api.Constant(2) will behaves as expected.
• Assign() method is removed, one can now simply assign witness value like so witness := &circuit{A: value}
• added api.IsConstant(Variable) -> bool and api.ConstantValue(Variable) -> big.Int which enables circuit developers to write specific version of their circuit depending on if a variable is known at compile time. Note: the big.Int test engine will return true to all values, and may not test all code path against the constraint solver.

Unsuspected side effect to this PR; frontend.Compile is now ~40% faster for Groth16.

[ivokub]

Looks good in general. Added a few type descriptions and fixed some assignments.