perf: api.IsZero generate less constraints

[gbotrel]

used to have 3 constraints for api.IsZero

// m * (1 - m) = 0       // constrain m to be 0 or 1
// a * m = 0            // constrain m to be 0 if a != 0
// _ = inverse(m + a) 	// constrain m to be 1 if a == 0
// m is computed by the solver such that m = 1 - a^(modulus - 1)

can be done in 2:

// x = 1/a 		// in a hint (x == 0 if a == 0)
// m = -a*x + 1         // constrain m to be 1 if a == 0
// a * m = 0            // constrain m to be 0 if a != 0