objtype: Rename mp_obj_class_t -> mp_obj_instance_t and move to local header.

TODO: Rename methods.

py/objtype.h

@@ -0,0 +1,8 @@
+// instance object
+// creating an instance of a class makes one of these objects
+typedef struct _mp_obj_instance_t {
+    mp_obj_base_t base;
+    mp_map_t members;
+    mp_obj_t subobj[];
+    // TODO maybe cache __getattr__ and __setattr__ for efficient lookup of them
+} mp_obj_instance_t;