py: Convert mp_uint_t to size_t for tuple/list accessors.

This patch changes mp_uint_t to size_t for the len argument of the
following public facing C functions:

mp_obj_tuple_get
mp_obj_list_get
mp_obj_get_array

These functions take a pointer to the len argument (to be filled in by the
function) and callers of these functions should update their code so the
type of len is changed to size_t.  For ports that don't use nan-boxing
there should be no change in generate code because the size of the type
remains the same (word sized), and in a lot of cases there won't even be a
compiler warning if the type remains as mp_uint_t.

The reason for this change is to standardise on the use of size_t for
variables that count memory (or memory related) sizes/lengths.  It helps
builds that use nan-boxing.

py/modthread.c

@@ -215,7 +215,7 @@ STATIC mp_obj_t mod_thread_start_new_thread(size_t n_args, const mp_obj_t *args)
     thread_entry_args_t *th_args;
 
     // get positional arguments
-    mp_uint_t pos_args_len;
+    size_t pos_args_len;
     mp_obj_t *pos_args_items;
     mp_obj_get_array(args[1], &pos_args_len, &pos_args_items);