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

py/objdict.c

@@ -577,8 +577,6 @@ const mp_obj_type_t mp_type_dict = {
 };
 
 #if MICROPY_PY_COLLECTIONS_ORDEREDDICT
-STATIC const mp_rom_obj_tuple_t ordereddict_base_tuple = {{&mp_type_tuple}, 1, {MP_ROM_PTR(&mp_type_dict)}};
-
 const mp_obj_type_t mp_type_ordereddict = {
     { &mp_type_type },
     .name = MP_QSTR_OrderedDict,
@@ -588,7 +586,7 @@ const mp_obj_type_t mp_type_ordereddict = {
     .binary_op = dict_binary_op,
     .subscr = dict_subscr,
     .getiter = dict_getiter,
-    .bases_tuple = (mp_obj_tuple_t*)(mp_rom_obj_tuple_t*)&ordereddict_base_tuple,
+    .parent = &mp_type_dict,
     .locals_dict = (mp_obj_dict_t*)&dict_locals_dict,
 };
 #endif