Move abs(int256) from Math to SafeMath (#3110)

Co-authored-by: Francisco Giordano <frangio.1@gmail.com>

contracts/utils/math/SignedMath.sol

@@ -29,4 +29,14 @@ library SignedMath {
         int256 x = (a & b) + ((a ^ b) >> 1);
         return x + (int256(uint256(x) >> 255) & (a ^ b));
     }
+
+    /**
+     * @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);
+        }
+    }
 }