Add test for io::Error's size.

library/std/src/io/error/tests.rs

 use super::{Custom, Error, ErrorKind, Repr};
 use crate::error;
 use crate::fmt;
+use crate::mem::size_of;
 use crate::sys::decode_error_kind;
 use crate::sys::os::error_string;
 
+#[test]
+fn test_size() {
+    assert!(size_of::<Error>() <= size_of::<[usize; 2]>());
+}
+
 #[test]
 fn test_debug_error() {
     let code = 6;