feat: use extcodesize for isContract to reduce gas (#2311)

* feat: use extcodesize for isContract to reduce gas

* feat: add changelog entry

CHANGELOG.md

 # Changelog
 
+## 3.1.1 (Unreleased)
+
+### Improvements
+ * `Address.isContract`: switched from `extcodehash` to `extcodesize` for less gas usage. ([#2311](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/2311))
+
 ## 3.1.0 (2020-06-23)
 
 ### New features

contracts/utils/Address.sol

@@ -24,14 +24,14 @@ library Address {
      * ====
      */
     function isContract(address account) internal view returns (bool) {
-        // According to EIP-1052, 0x0 is the value returned for not-yet created accounts
-        // and 0xc5d2460186f7233c927e7db2dcc703c0e500b653ca82273b7bfad8045d85a470 is returned
-        // for accounts without code, i.e. `keccak256('')`
-        bytes32 codehash;
-        bytes32 accountHash = 0xc5d2460186f7233c927e7db2dcc703c0e500b653ca82273b7bfad8045d85a470;
+        // This method relies in extcodesize, which returns 0 for contracts in
+        // construction, since the code is only stored at the end of the
+        // constructor execution.
+
+        uint256 size;
         // solhint-disable-next-line no-inline-assembly
-        assembly { codehash := extcodehash(account) }
-        return (codehash != accountHash && codehash != 0x0);
+        assembly { size := extcodesize(account) }
+        return size > 0;
     }
 
     /**