py/objtype: Allow passing keyword arguments to native base __init__.

Allowing passing keyword arguments to a native base's __init__, i.e.
`make_new` in the C code.  Previously only positional arguments were
allowed.

The main trade-off in this commit is that every call to the native base's
`make_new` is now going to be preceded by a call to
`mp_map_init_fixed_table` even though most of what that does is unused and
instead it merely serves as a way to pass the number of keyword arguments.

Fixes issue #15465.

Signed-off-by: stijn <stijn@ignitron.net>

py/obj.h

@@ -516,6 +516,9 @@ typedef mp_obj_t (*mp_fun_3_t)(mp_obj_t, mp_obj_t, mp_obj_t);
 typedef mp_obj_t (*mp_fun_var_t)(size_t n, const mp_obj_t *);
 // mp_fun_kw_t takes mp_map_t* (and not const mp_map_t*) to ease passing
 // this arg to mp_map_lookup().
+// Note that the mp_obj_t* array will contain all arguments, positional and keyword, with the keyword
+// ones starting at offset n, like: arg0 arg1 ... arg<n> key0 value0 key1 value1 ..., and the mp_map_t*
+// gets those same keyword arguments but as a map for convenience; see fun_builtin_var_call.
 typedef mp_obj_t (*mp_fun_kw_t)(size_t n, const mp_obj_t *, mp_map_t *);
 
 // Flags for type behaviour (mp_obj_type_t.flags)