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/utils/math/Math.sol

@@ -40,4 +40,14 @@ library Math {
         // (a + b - 1) / b can overflow on addition, so we distribute.
         return a / b + (a % b == 0 ? 0 : 1);
     }
+
+    /**
+     * @dev Returns the absolute unsigned value of a signed value.
+     */
+    function abs(int256 n) internal pure returns (uint256) {
+        unchecked {
+            // must be unchecked in order to support `n = type(int256).min`
+            return uint256(n >= 0 ? n : -n);
+        }
+    }
 }