Set error code sign correctly (#175)

* fix: set error code sign correctly
* chore: add comment with link to source code

contracts/v0.8/utils/Actor.sol

@@ -72,9 +72,9 @@ library Actor {
     }
 
     function getErrorCodeMsg(int256 exit_code) internal pure returns (string memory) {
-        if (exit_code < 0) {
-            return string.concat("actor error code -", Strings.toString(uint256(exit_code)));
-        }
-        return string.concat("actor error code -", Strings.toString(uint256(exit_code)));
+        return
+            exit_code >= 0
+                ? string.concat("actor error code ", Strings.toString(uint256(exit_code)))
+                : string.concat("actor error code -", Strings.toString(Misc.abs(exit_code)));
     }
 }

contracts/v0.8/utils/Misc.sol

@@ -22,56 +22,12 @@ library Misc {
     uint64 constant CBOR_CODEC = 0x71;
     uint64 constant NONE_CODEC = 0x00;
 
-    function toUint256(bytes32 _bytes) internal pure returns (uint256 value) {
-        return uint256(_bytes);
-    }
-
-    function toInt256(bytes32 _bytes) internal pure returns (int256 value) {
-        return int256(uint256(_bytes));
-    }
-
-    function toUint256(bytes memory _bytes, uint offset) internal pure returns (uint256 value) {
-        assembly {
-            value := mload(add(_bytes, offset))
-        }
-    }
-
-    function toInt256(bytes memory _bytes, uint offset) internal pure returns (int256 value) {
-        return int256(toUint256(_bytes, offset));
-    }
-
-    function toBytes(uint256 x) internal pure returns (bytes memory b) {
-        b = new bytes(32);
-        assembly {
-            mstore(add(b, 32), x)
-        }
-    }
-
-    function toBytes(int256 x) internal pure returns (bytes memory b) {
-        b = new bytes(32);
-        assembly {
-            mstore(add(b, 32), x)
-        }
-    }
-
-    function copy(uint src, uint dest, uint len) internal pure {
-        // Copy word-length chunks while possible
-        for (; len >= 32; len -= 32) {
-            assembly {
-                mstore(dest, mload(src))
-            }
-            dest += 32;
-            src += 32;
-        }
-
-        if (len == 0) return;
-
-        // Copy remaining bytes
-        uint mask = 256 ** (32 - len) - 1;
-        assembly {
-            let srcpart := and(mload(src), not(mask))
-            let destpart := and(mload(dest), mask)
-            mstore(dest, or(destpart, srcpart))
+    // Code taken from Openzeppelin repo
+    // Link: https://github.com/OpenZeppelin/openzeppelin-contracts/blob/0320a718e8e07b1d932f5acb8ad9cec9d9eed99b/contracts/utils/math/SignedMath.sol#L37-L42
+    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);
         }
     }
 }