Add a simple wrapper for address. (#1773)

* Updated code style to no more than120 characters per line.

* Unify code comments style with Doxygen-style tags.

* Fix the conflicts.

* Add a return value in the contract ERC20Burnable.

* A Add a wrapper function to change type of address to address payable.

* U Modify Address utils.

* A Add test case for Address.

* U Modify code style in ERC20Burnable.

* Add changelog entry.

* Improved dev docs.

contracts/mocks/AddressImpl.sol

@@ -6,4 +6,8 @@ contract AddressImpl {
     function isContract(address account) external view returns (bool) {
         return Address.isContract(account);
     }
+
+    function toPayable(address account) external pure returns (address payable) {
+        return Address.toPayable(account);
+    }
 }

contracts/utils/Address.sol

@@ -24,4 +24,12 @@ library Address {
         assembly { size := extcodesize(account) }
         return size > 0;
     }
+
+    /**
+     * @dev Converts an `address` into `address payable`. Note that this is
+     * simply a type cast: the actual underlying value is not changed.
+     */
+    function toPayable(address account) internal pure returns (address payable) {
+        return address(uint160(account));
+    }
 }