fix: invmod returning a value outside the range of a negative modulus

[kevin9327]

invmod gives a wrong-range result when the modulus is negative:

math.invmod(3, -11) // -18, expected -7
math.invmod(2, -7)  // -10, expected -3

mod() already returns a value with the sign of b, so after inv = mod(inv, b) the result lies between 0 and b. The extra if (inv < 0) inv = add(inv, b) step can only fire for a negative b, and then it moves the value out of that range. (-18 is still congruent, but it's not the reduced inverse, and it doesn't match what mod returns.) For a positive b the branch never runs, so removing it changes nothing there.

The new results match Python's pow(a, -1, m), e.g. pow(3, -1, -11) == -7.

This PR removes that step and the smaller/add dependencies that are no longer used, and adds tests for a negative modulus with numbers and BigNumbers. npx mocha test/unit-tests/function/arithmetic/ passes and eslint is clean.