rework erc20

start to standardize syntax

4 spaces/tab

add implementation reference links

improve tests

contracts/libraries/Math.sol

@@ -0,0 +1,21 @@
+pragma solidity 0.5.12;
+
+library Math {
+    // based on https://github.com/OpenZeppelin/openzeppelin-contracts/blob/2f9ae975c8bdc5c7f7fa26204896f6c717f07164/contracts/math/Math.sol#L17
+    function min(uint256 a, uint256 b) internal pure returns (uint256) {
+        return a < b ? a : b;
+    }
+
+    // based on https://github.com/ethereum/dapp-bin/blob/11f05fc9e3f31a00d57982bc2f65ef2654f1b569/library/math.sol#L28 via https://github.com/ethereum/dapp-bin/pull/50
+    function sqrt(uint256 x) internal pure returns (uint256) {
+        if (x == 0) return 0;
+        else if (x <= 3) return 1;
+        uint256 z = (x + 1) / 2;
+        uint256 y = x;
+        while (z < y) {
+            y = z;
+            z = (x / z + z) / 2;
+        }
+        return y;
+    }
+}

contracts/libraries/SafeMath.sol

@@ -0,0 +1,29 @@
+// based on https://github.com/OpenZeppelin/openzeppelin-contracts/blob/2f9ae975c8bdc5c7f7fa26204896f6c717f07164/contracts/math/SafeMath.sol
+pragma solidity 0.5.12;
+
+library SafeMath {
+    function add(uint256 a, uint256 b) internal pure returns (uint256) {
+        uint256 c = a + b;
+        require(c >= a, "SafeMath: addition overflow");
+        return c;
+    }
+
+    function sub(uint256 a, uint256 b) internal pure returns (uint256) {
+        require(b <= a, "SafeMath: subtraction overflow");
+        uint256 c = a - b;
+        return c;
+    }
+
+    function mul(uint256 a, uint256 b) internal pure returns (uint256) {
+        if (a == 0) return 0;
+        uint256 c = a * b;
+        require(c / a == b, "SafeMath: multiplication overflow");
+        return c;
+    }
+
+    function div(uint256 a, uint256 b) internal pure returns (uint256) {
+        require(b > 0, "SafeMath: division by zero");
+        uint256 c = a / b;
+        return c;
+    }
+}