py/vm: Add a fast path for LOAD_ATTR on instance types.

When the LOAD_ATTR opcode is executed there are quite a few different cases
that have to be handled, but the common case is accessing a member on an
instance type.  Typically, built-in types provide methods which is why this
is common.

Fortunately, for this specific case, if the member is found in the member
map then there's no further processing.

This optimisation does a relatively cheap check (type is instance) and then
forwards directly to the member map lookup, falling back to the regular
path if necessary.

Signed-off-by: Jim Mussared <jim.mussared@gmail.com>

py/objtype.c

@@ -579,6 +579,7 @@ STATIC void mp_obj_instance_load_attr(mp_obj_t self_in, qstr attr, mp_obj_t *des
     assert(mp_obj_is_instance_type(mp_obj_get_type(self_in)));
     mp_obj_instance_t *self = MP_OBJ_TO_PTR(self_in);
 
+    // Note: This is fast-path'ed in the VM for the MP_BC_LOAD_ATTR operation.
     mp_map_elem_t *elem = mp_map_lookup(&self->members, MP_OBJ_NEW_QSTR(attr), MP_MAP_LOOKUP);
     if (elem != NULL) {
         // object member, always treated as a value