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.
diff --git a/py/obj.c b/py/obj.c
index 4b5fe25..98ffa93 100644
--- a/py/obj.c
+++ b/py/obj.c
@@ -321,7 +321,7 @@
 #endif
 
 // note: returned value in *items may point to the interior of a GC block
-void mp_obj_get_array(mp_obj_t o, mp_uint_t *len, mp_obj_t **items) {
+void mp_obj_get_array(mp_obj_t o, size_t *len, mp_obj_t **items) {
     if (MP_OBJ_IS_TYPE(o, &mp_type_tuple)) {
         mp_obj_tuple_get(o, len, items);
     } else if (MP_OBJ_IS_TYPE(o, &mp_type_list)) {
@@ -338,7 +338,7 @@
 
 // note: returned value in *items may point to the interior of a GC block
 void mp_obj_get_array_fixed_n(mp_obj_t o, size_t len, mp_obj_t **items) {
-    mp_uint_t seq_len;
+    size_t seq_len;
     mp_obj_get_array(o, &seq_len, items);
     if (seq_len != len) {
         if (MICROPY_ERROR_REPORTING == MICROPY_ERROR_REPORTING_TERSE) {