Addition of Or,And constraint, plus a specific constraint checking if a value is zero

[ThomasPiellard]

Fixes #45

Addition of Or, And, IsZero constraints.

Example:

Referring to the use in #45 here is an example:

` type ExampleCircuit struct {
X frontend.Variable
}

func (circuit *ExampleCircuit) Define(curveID gurvy.ID, cs *frontend.ConstraintSystem) error {

	XminusThree := cs.Sub(circuit.X, 3)
	XminusFour := cs.Sub(circuit.X, 4)

	isXEqualToThree := cs.IsZero(XminusThree, curveID)
	isXEqualToFour := cs.IsZero(XminusFour, curveID)

	xEqualThreeOrFour := cs.Or(isXEqualToThree, isXEqualToFour)

	cs.Println("x==3?: ", isXEqualToThree)
	cs.Println("x==4?: ", isXEqualToFour)
	cs.Println("x==3 or x==4?: ", xEqualThreeOrFour)

	cs.AssertIsEqual(xEqualThreeOrFour, 1)

	return nil
}

func main() {

	var circuit ExampleCircuit
	r1cs, _ := frontend.Compile(gurvy.BN256, &circuit)

	var solution ExampleCircuit
	solution.X.Assign(3)
	parsedSolution, _ := frontend.ParseWitness(&solution)

	r1cs.IsSolved(parsedSolution)

}

`

Output:
main.go:26 x==3?: 1
main.go:27 x==4?: 0
main.go:28 x==3 or x==4?: 1

Status: tests pass.