Unsigned conversion #2111 (#2123)

* Add signed to unsigned conversion to SafeCast

* Update SafeCast exception message

* Add test for SafeCast int to uint conversion

- Update SafeCastMock
- Add tests for SafeCast int256 to uint256

* Update SafeCast int to uint definition

Apply suggestions from code review.

Co-Authored-By: Nicolás Venturo <nicolas.venturo@gmail.com>

* Update test for SafeCast int to uint conversion

* Update SafeCast test after code review

- Change "downcasts" to "casts"
- Move test closer to its function

* Fix error in SafeCast toUint256 description

* Fix breaking error in SafeCast

* Add uint256 to int256 conversion to SafeCast

- Add function
- Add mock
- Add test

* Update SafeCast unsigned to signed conversion

- Update error in conversion to be more clear
- Update constants in test to be powers of 2 instead of shifts

* Add changelog entry

* Update SafeCast tests

- Add minus in INT256_MIN for clarity

Co-Authored-By: Nicolás Venturo <nicolas.venturo@gmail.com>

Co-authored-by: Nicolás Venturo <nicolas.venturo@gmail.com>

contracts/mocks/SafeCastMock.sol

@@ -4,6 +4,15 @@ import "../utils/SafeCast.sol";
 
 contract SafeCastMock {
     using SafeCast for uint;
+    using SafeCast for int;
+
+    function toUint256(int a) public pure returns (uint256) {
+        return a.toUint256();
+    }
+
+    function toInt256(uint a) public pure returns (int256) {
+        return a.toInt256();
+    }
 
     function toUint128(uint a) public pure returns (uint128) {
         return a.toUint128();

contracts/utils/SafeCast.sol

@@ -92,4 +92,28 @@ library SafeCast {
         require(value < 2**8, "SafeCast: value doesn\'t fit in 8 bits");
         return uint8(value);
     }
+
+    /**
+     * @dev Converts a signed int256 into an unsigned uint256.
+     *
+     * Requirements:
+     *
+     * - input must be greater than or equal to 0.
+     */
+    function toUint256(int256 value) internal pure returns (uint256) {
+        require(value >= 0, "SafeCast: value must be positive");
+        return uint256(value);
+    }
+
+    /**
+     * @dev Converts an unsigned uint256 into a signed int256.
+     *
+     * Requirements:
+     *
+     * - input must be less than or equal to maxInt256.
+     */
+    function toInt256(uint256 value) internal pure returns (int256) {
+        require(value < 2**255, "SafeCast: value doesn't fit in an int256");
+        return int256(value);
+    }
 }