Reorganize the repo structure (#2503)

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

contracts/utils/math/SignedSafeMath.sol

@@ -0,0 +1,67 @@
+// SPDX-License-Identifier: MIT
+
+pragma solidity ^0.8.0;
+
+/**
+ * @dev Wrappers over Solidity's arithmetic operations.
+ *
+ * NOTE: `SignedSafeMath` is no longer needed starting with Solidity 0.8. The compiler
+ * now has built in overflow checking.
+ */
+library SignedSafeMath {
+    /**
+     * @dev Returns the multiplication of two signed integers, reverting on
+     * overflow.
+     *
+     * Counterpart to Solidity's `*` operator.
+     *
+     * Requirements:
+     *
+     * - Multiplication cannot overflow.
+     */
+    function mul(int256 a, int256 b) internal pure returns (int256) {
+        return a * b;
+    }
+
+    /**
+     * @dev Returns the integer division of two signed integers. Reverts on
+     * division by zero. The result is rounded towards zero.
+     *
+     * Counterpart to Solidity's `/` operator.
+     *
+     * Requirements:
+     *
+     * - The divisor cannot be zero.
+     */
+    function div(int256 a, int256 b) internal pure returns (int256) {
+        return a / b;
+    }
+
+    /**
+     * @dev Returns the subtraction of two signed integers, reverting on
+     * overflow.
+     *
+     * Counterpart to Solidity's `-` operator.
+     *
+     * Requirements:
+     *
+     * - Subtraction cannot overflow.
+     */
+    function sub(int256 a, int256 b) internal pure returns (int256) {
+        return a - b;
+    }
+
+    /**
+     * @dev Returns the addition of two signed integers, reverting on
+     * overflow.
+     *
+     * Counterpart to Solidity's `+` operator.
+     *
+     * Requirements:
+     *
+     * - Addition cannot overflow.
+     */
+    function add(int256 a, int256 b) internal pure returns (int256) {
+        return a + b;
+    }
+}