Apply suggestions from code review

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

contracts/utils/math/SafeCast.sol

@@ -131,7 +131,7 @@ library SafeCast {
      * - input must be greater than or equal to 0.
      */
     function toUint256(int256 value) internal pure returns (uint256) {
-        require(value >= int256(type(uint256).min), "SafeCast: value must be positive");
+        require(value >= 0, "SafeCast: value must be positive");
         return uint256(value);
     }
 
@@ -233,6 +233,7 @@ library SafeCast {
      * - input must be less than or equal to maxInt256.
      */
     function toInt256(uint256 value) internal pure returns (int256) {
+        // Note: Unsafe cast below is okay because `type(int256).max` is guaranteed to be positive
         require(value <= uint256(type(int256).max), "SafeCast: value doesn't fit in an int256");
         return int256(value);
     }