cs.IsZero is not efficient

[gbotrel]

cs.IsZero(a) returns 1 if a == 0 and 0 otherwise.

Replace current circuit code by these 3 constraints (instead of binary decomposition):

m * (1 - m) = 0       // constrain m to be 0 or 1
m * a = 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)