Fix zero sizing of Iter<T>

---- test_iter stdout ----
thread 'test_iter' panicked at tests/test.rs:7:5:
assertion `left == right` failed
  left: 0
 right: 8
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

failures:
    test_iter

It seems https://github.com/rust-lang/rust/pull/115277 in nightly-2023-08-30 affected the size of this type as originally written.

dtolnay / inventory

Fix zero sizing of Iter<T> #63