Add Strings.equal (#3774)

Co-authored-by: Francisco <fg@frang.io>

contracts/utils/Strings.sol

@@ -67,4 +67,11 @@ library Strings {
     function toHexString(address addr) internal pure returns (string memory) {
         return toHexString(uint256(uint160(addr)), _ADDRESS_LENGTH);
     }
+
+    /**
+     * @dev Returns true if the two strings are equal.
+     */
+    function equal(string memory a, string memory b) internal pure returns (bool) {
+        return keccak256(bytes(a)) == keccak256(bytes(b));
+    }
 }