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/objstr.c b/py/objstr.c
index 4053efb..682bd26 100644
--- a/py/objstr.c
+++ b/py/objstr.c
@@ -284,7 +284,7 @@
     // check for modulo
     if (op == MP_BINARY_OP_MODULO) {
         mp_obj_t *args = &rhs_in;
-        mp_uint_t n_args = 1;
+        size_t n_args = 1;
         mp_obj_t dict = MP_OBJ_NULL;
         if (MP_OBJ_IS_TYPE(rhs_in, &mp_type_tuple)) {
             // TODO: Support tuple subclasses?
@@ -428,7 +428,7 @@
     GET_STR_DATA_LEN(self_in, sep_str, sep_len);
 
     // process args
-    mp_uint_t seq_len;
+    size_t seq_len;
     mp_obj_t *seq_items;
 
     if (!MP_OBJ_IS_TYPE(arg, &mp_type_list) && !MP_OBJ_IS_TYPE(arg, &mp_type_tuple)) {