py: Convert [u]int to mp_[u]int_t where appropriate.

Addressing issue #50.
diff --git a/py/modstruct.c b/py/modstruct.c
index 8d44006..8122a96 100644
--- a/py/modstruct.c
+++ b/py/modstruct.c
@@ -164,7 +164,7 @@
     // TODO: "The arguments must match the values required by the format exactly."
     const char *fmt = mp_obj_str_get_str(args[0]);
     char fmt_type = get_fmt_type(&fmt);
-    int size = MP_OBJ_SMALL_INT_VALUE(struct_calcsize(args[0]));
+    mp_int_t size = MP_OBJ_SMALL_INT_VALUE(struct_calcsize(args[0]));
     byte *p;
     mp_obj_t res = mp_obj_str_builder_start(&mp_type_bytes, size, &p);
     memset(p, 0, size);