patch 9.0.1152: class "implements" argument not implemented

Problem:    Class "implements" argument not implemented.
Solution:   Implement "implements" argument.  Add basic checks for when a
            class implements an interface.

src/structs.h

@@ -1494,6 +1494,10 @@ struct class_S
     int		class_refcount;
     int		class_copyID;		// used by garbage collection
 
+    // interfaces declared for the class
+    int		class_interface_count;
+    char_u	**class_interfaces;	// allocated array of names
+
     // class members: "static varname"
     int		class_class_member_count;
     ocmember_T	*class_class_members;	// allocated