Use Iterator#product instead of explicit impl.
Use Iterator#product instead of explicit impl.

Since Rust 1.11 the iterator product method is stable now, so this
changeset switches over to the rust implementation. It keeps the
surrounding method since otherwise noisy type declarations need
to be provided in many places.

tensorflow/rust,adamcrume/tensorflow-rust,adamcrume/tensorflow-rust,tensorflow/rust,adamcrume/tensorflow-rust,adamcrume/tensorflow-rust,tensorflow/rust,tensorflow/rust