py: Optimise types for common case where type has a single parent type.

The common cases for inheritance are 0 or 1 parent types, for both built-in
types (eg built-in exceptions) as well as user defined types.  So it makes
sense to optimise the case of 1 parent type by storing just the type and
not a tuple of 1 value (that value being the single parent type).

This patch makes such an optimisation.  Even though there is a bit more
code to handle the two cases (either a single type or a tuple with 2 or
more values) it helps reduce overall code size because it eliminates the
need to create a static tuple to hold single parents (eg for the built-in
exceptions).  It also helps reduce RAM usage for user defined types that
only derive from a single parent.

Changes in code size (in bytes) due to this patch:

    bare-arm:       -16
    minimal (x86): -176
    unix (x86-64): -320
    unix nanbox:   -384
    stmhal:         -64
    cc3200:         -32
    esp8266:       -108

1 files changed, 1 insertions, 3 deletions

diff --git a/py/objnamedtuple.c b/py/objnamedtuple.c
index 52468747b..cbd845dce 100644
--- a/py/objnamedtuple.c
+++ b/py/objnamedtuple.c
@@ -134,8 +134,6 @@ STATIC mp_obj_t namedtuple_make_new(const mp_obj_type_t *type_in, size_t n_args,
     return MP_OBJ_FROM_PTR(tuple);
 }
 
-STATIC const mp_rom_obj_tuple_t namedtuple_base_tuple = {{&mp_type_tuple}, 1, {MP_ROM_PTR(&mp_type_tuple)}};
-
 STATIC mp_obj_t mp_obj_new_namedtuple_type(qstr name, size_t n_fields, mp_obj_t *fields) {
     mp_obj_namedtuple_type_t *o = m_new_obj_var(mp_obj_namedtuple_type_t, qstr, n_fields);
     memset(&o->base, 0, sizeof(o->base));
@@ -148,7 +146,7 @@ STATIC mp_obj_t mp_obj_new_namedtuple_type(qstr name, size_t n_fields, mp_obj_t
     o->base.attr = namedtuple_attr;
     o->base.subscr = mp_obj_tuple_subscr;
     o->base.getiter = mp_obj_tuple_getiter;
-    o->base.bases_tuple = (mp_obj_tuple_t*)(mp_rom_obj_tuple_t*)&namedtuple_base_tuple;
+    o->base.parent = &mp_type_tuple;
     o->n_fields = n_fields;
     for (size_t i = 0; i < n_fields; i++) {
         o->fields[i] = mp_obj_str_get_qstr(fields[i]);