py: Wrap all obj-ptr conversions in MP_OBJ_TO_PTR/MP_OBJ_FROM_PTR.

This allows the mp_obj_t type to be configured to something other than a
pointer-sized primitive type.

This patch also includes additional changes to allow the code to compile
when sizeof(mp_uint_t) != sizeof(void*), such as using size_t instead of
mp_uint_t, and various casts.
diff --git a/py/modstruct.c b/py/modstruct.c
index fb206bc..a52c507 100644
--- a/py/modstruct.c
+++ b/py/modstruct.c
@@ -137,7 +137,7 @@
     const char *fmt = mp_obj_str_get_str(fmt_in);
     char fmt_type = get_fmt_type(&fmt);
     uint size = calcsize_items(fmt);
-    mp_obj_tuple_t *res = mp_obj_new_tuple(size, NULL);
+    mp_obj_tuple_t *res = MP_OBJ_TO_PTR(mp_obj_new_tuple(size, NULL));
     mp_buffer_info_t bufinfo;
     mp_get_buffer_raise(data_in, &bufinfo, MP_BUFFER_READ);
     byte *p = bufinfo.buf;
@@ -161,7 +161,7 @@
         }
         fmt++;
     }
-    return res;
+    return MP_OBJ_FROM_PTR(res);
 }
 MP_DEFINE_CONST_FUN_OBJ_2(struct_unpack_obj, struct_unpack);