A function which returns the absolute value of a signed value (#2984)

* Add a function which returns the absolute (and obviously unsigned) value of a signed value.

* add changelog entry and fix lint

Co-authored-by: Hadrien Croubois <hadrien.croubois@gmail.com>

contracts/mocks/MathMock.sol

@@ -20,4 +20,8 @@ contract MathMock {
     function ceilDiv(uint256 a, uint256 b) public pure returns (uint256) {
         return Math.ceilDiv(a, b);
     }
+
+    function abs(int256 n) public pure returns (uint256) {
+        return Math.abs(n);
+    }
 }