Fix/xor cst var plonk

[ThomasPiellard]

Changes:

• Addition of tests for Xor( var, )
• Simplified the writing of Xor in the r1cs API
• Fixed the Xor in sparseR1cs when one of the operands is constant

in the reduce method of the r1cs, the terms multiplied by zeroes are removed:

	// if the coefficient is zero, we remove it
        if ccID == compiled.CoeffIdZero {
	        l = append(l[:i], l[i+1:]...)
	        continue
        }