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/objtuple.c

@@ -244,7 +244,7 @@ mp_obj_t mp_obj_new_tuple(size_t n, const mp_obj_t *items) {
     return MP_OBJ_FROM_PTR(o);
 }
 
-void mp_obj_tuple_get(mp_obj_t self_in, mp_uint_t *len, mp_obj_t **items) {
+void mp_obj_tuple_get(mp_obj_t self_in, size_t *len, mp_obj_t **items) {
     assert(MP_OBJ_IS_TYPE(self_in, &mp_type_tuple));
     mp_obj_tuple_t *self = MP_OBJ_TO_PTR(self_in);
     *len = self->len;