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

* use prefix underscore for internal state variables

* make _supportedInterfaces private

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;
     }
     ```
 

contracts/introspection/ERC165.sol

@@ -18,7 +18,7 @@ contract ERC165 is IERC165 {
   /**
    * @dev a mapping of interface id to whether or not it's supported
    */
-  mapping(bytes4 => bool) internal _supportedInterfaces;
+  mapping(bytes4 => bool) private _supportedInterfaces;
 
   /**
    * @dev A contract implementing SupportsInterfaceWithLookup

contracts/mocks/ERC165/ERC165InterfacesSupported.sol

@@ -21,7 +21,7 @@ contract SupportsInterfaceWithLookupMock is IERC165 {
   /**
    * @dev a mapping of interface id to whether or not it's supported
    */
-  mapping(bytes4 => bool) internal supportedInterfaces;
+  mapping(bytes4 => bool) private _supportedInterfaces;
 
   /**
    * @dev A contract implementing SupportsInterfaceWithLookup
@@ -41,7 +41,7 @@ contract SupportsInterfaceWithLookupMock is IERC165 {
     view
     returns (bool)
   {
-    return supportedInterfaces[interfaceId];
+    return _supportedInterfaces[interfaceId];
   }
 
   /**
@@ -51,7 +51,7 @@ contract SupportsInterfaceWithLookupMock is IERC165 {
     internal
   {
     require(interfaceId != 0xffffffff);
-    supportedInterfaces[interfaceId] = true;
+    _supportedInterfaces[interfaceId] = true;
   }
 }