Add Strings.toString for signed integers (#3773)

Co-authored-by: Hadrien Croubois <hadrien.croubois@gmail.com>
Co-authored-by: Gbolahan <89295500+GbolahanAnon@users.noreply.github.com>
Co-authored-by: Francisco Giordano <frangio.1@gmail.com>

contracts/mocks/StringsMock.sol

@@ -9,6 +9,10 @@ contract StringsMock {
         return Strings.toString(value);
     }
 
+    function toString(int256 value) public pure returns (string memory) {
+        return Strings.toString(value);
+    }
+
     function toHexString(uint256 value) public pure returns (string memory) {
         return Strings.toHexString(value);
     }

contracts/utils/Strings.sol

@@ -4,6 +4,7 @@
 pragma solidity ^0.8.0;
 
 import "./math/Math.sol";
+import "./math/SignedMath.sol";
 
 /**
  * @dev String operations.
@@ -37,6 +38,13 @@ library Strings {
         }
     }
 
+    /**
+     * @dev Converts a `int256` to its ASCII `string` decimal representation.
+     */
+    function toString(int256 value) internal pure returns (string memory) {
+        return string(abi.encodePacked(value < 0 ? "-" : "", toString(SignedMath.abs(value))));
+    }
+
     /**
      * @dev Converts a `uint256` to its ASCII `string` hexadecimal representation.
      */