Add library implementation of SafeMath

contracts/utils/contracts/src/LibSafeMath.sol

@@ -0,0 +1,83 @@
+pragma solidity ^0.5.9;
+
+import "./LibRichErrors.sol";
+import "./LibSafeMathRichErrors.sol";
+
+
+library LibSafeMath {
+
+    function safeMul(uint256 a, uint256 b)
+        internal
+        pure
+        returns (uint256)
+    {
+        if (a == 0) {
+            return 0;
+        }
+        uint256 c = a * b;
+        if (c / a != b) {
+            LibRichErrors._rrevert(LibSafeMathRichErrors.SafeMathError(
+                LibSafeMathRichErrors.SafeMathErrorCodes.UINT256_MULTIPLICATION_OVERFLOW,
+                a,
+                b
+            ));
+        }
+        return c;
+    }
+
+    function safeDiv(uint256 a, uint256 b)
+        internal
+        pure
+        returns (uint256)
+    {
+        uint256 c = a / b;
+        return c;
+    }
+
+    function safeSub(uint256 a, uint256 b)
+        internal
+        pure
+        returns (uint256)
+    {
+        if (b > a) {
+            LibRichErrors._rrevert(LibSafeMathRichErrors.SafeMathError(
+                LibSafeMathRichErrors.SafeMathErrorCodes.UINT256_SUBTRACTION_UNDERFLOW,
+                a,
+                b
+            ));
+        }
+        return a - b;
+    }
+
+    function safeAdd(uint256 a, uint256 b)
+        internal
+        pure
+        returns (uint256)
+    {
+        uint256 c = a + b;
+        if (c < a) {
+            LibRichErrors._rrevert(LibSafeMathRichErrors.SafeMathError(
+                LibSafeMathRichErrors.SafeMathErrorCodes.UINT256_ADDITION_OVERFLOW,
+                a,
+                b
+            ));
+        }
+        return c;
+    }
+
+    function max256(uint256 a, uint256 b)
+        internal
+        pure
+        returns (uint256)
+    {
+        return a >= b ? a : b;
+    }
+
+    function min256(uint256 a, uint256 b)
+        internal
+        pure
+        returns (uint256)
+    {
+        return a < b ? a : b;
+    }
+}