optimise sub (#280)

Replacing 2 subs or 1 add + 1 sub
with
1 sub or 1 add plus 1 sub

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## Pull Request type

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pull requests if needed. -->

Minor sub optimise, figured this recently...

Please check the type of change your PR introduces:

- [ ] Bugfix
- [ ] Feature
- [ ] Code style update (formatting, renaming)
- [x] Refactoring (no functional changes, no API changes)
- [ ] Build-related changes
- [ ] Documentation content changes
- [ ] Other (please describe):

## What is the current behavior?

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link to a relevant issue. -->

Subtract does unnecessary ops

Issue Number: N/A

## What is the new behavior?

Comparison operator is subtraction behind the scenes, when we compare a
and b, we don't need to do the sub again.
`if` is replaced with a `match` that just returns the diff right away
when there's no overflow (it'll be much better for non worst case
branch).

```diff
- test alexandria_math::tests::mod_arithmetics_test::sub_mod_1_test ... ok (gas usage est.: 678590)
+ test alexandria_math::tests::mod_arithmetics_test::sub_mod_1_test ... ok (gas usage est.: 615870)
- test alexandria_math::tests::mod_arithmetics_test::sub_mod_2_test ... ok (gas usage est.: 678590)
+ test alexandria_math::tests::mod_arithmetics_test::sub_mod_2_test ... ok (gas usage est.: 615870)
- test alexandria_math::tests::mod_arithmetics_test::sub_mod_test ... ok (gas usage est.: 703270)
+ test alexandria_math::tests::mod_arithmetics_test::sub_mod_test ... ok (gas usage est.: 640550)
```

## Does this introduce a breaking change?

- [ ] Yes
- [*] No

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impact and migration path for existing applications below. -->

## Other information

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screenshots of how the component looks before and after the change. -->

src/math/src/mod_arithmetics.cairo

@@ -57,10 +57,14 @@ fn sub_mod(mut a: u256, mut b: u256, modulo: u256) -> u256 {
     // reduce values
     a = a % modulo;
     b = b % modulo;
-    if (a >= b) {
-        return a - b;
+    let (diff, overflow) = integer::u256_overflow_sub(a, b);
+    if overflow {
+        // Overflow back with add modulo
+        let (diff, _) = integer::u256_overflowing_add(diff, modulo);
+        diff
+    } else {
+        diff
     }
-    a + add_inverse_mod(b, modulo)
 }
 
 /// Function that performs modular multiplication.