Improve encapsulation on ERC165 and update code style guide (#1379)

* use prefix underscore for internal state variables

* make _supportedInterfaces private

(cherry picked from commit 03dfb2965c)

CODE_STYLE.md

@@ -23,6 +23,7 @@ Any exception or additions specific to our project are documented below.
     ```
     contract TestContract {
       uint256 private _privateVar;
+      uint256 internal _internalVar;
     }
     ```