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/objnamedtuple.c

@@ -158,7 +158,7 @@ STATIC mp_obj_t mp_obj_new_namedtuple_type(qstr name, size_t n_fields, mp_obj_t
 
 STATIC mp_obj_t new_namedtuple_type(mp_obj_t name_in, mp_obj_t fields_in) {
     qstr name = mp_obj_str_get_qstr(name_in);
-    mp_uint_t n_fields;
+    size_t n_fields;
     mp_obj_t *fields;
     #if MICROPY_CPYTHON_COMPAT
     if (MP_OBJ_IS_STR(fields_in)) {