TODO: Store NOTHING_UP_MY_SLEEVE.neg() as a constant

[weijiekoh]

In Semacaulk.sol, insertIdentity() computes the negative of the NUMS value in the field:

uint256 n = Constants.PRIME_R;
uint256 negZero = mulmod(NOTHING_UP_MY_SLEEVE_ZERO, n - 1, n);

Instead, negZero can be stored as a constant or computed in the constructor and stored as an immutable.