Added Cairo inv_mod function

[yuvalsw]

This change is 

[orizi]

Reviewed 2 of 2 files at r1, all commit messages.
Reviewable status: all files reviewed, 2 unresolved discussions (waiting on @yuvalsw)

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corelib/src/math.cairo line 63 at r1 (raw file):

        if sub_direction {
            let (_, inv) = TDivRemImpl::div_rem(s, n);
            Option::Some(inv)

Suggestion:

            // Extended gcd returns `s` <= [n / 2*g] - so it is already in range.
            Option::Some(s)

---

corelib/src/math.cairo line 68 at r1 (raw file):

            // sufficient to add `n` to make sure we have an unsigned integer.
            let (_, inv) = TDivRemImpl::div_rem(n.into() - s, n);
            Option::Some(inv)

Suggestion:

            // The absolute values of Bezout coefficients are guaranteed to be `< n`, 
            // so it will never be 0, so it is sufficient to return `n - s` to get a number in the correct range.
            Option::Some(n.into() - s)

[yuvalsw]

Reviewable status: 0 of 6 files reviewed, 2 unresolved discussions (waiting on @orizi)

---

corelib/src/math.cairo line 63 at r1 (raw file):

        if sub_direction {
            let (_, inv) = TDivRemImpl::div_rem(s, n);
            Option::Some(inv)

Done.

---

corelib/src/math.cairo line 68 at r1 (raw file):

            // sufficient to add `n` to make sure we have an unsigned integer.
            let (_, inv) = TDivRemImpl::div_rem(n.into() - s, n);
            Option::Some(inv)

Done.

[orizi]

Reviewed 6 of 6 files at r5, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @yuvalsw)