add examples for shift_elements_{left, right} and rotate_elements_{left, right}

[folkertdev]

I think the text is just complicated enough to warrant an example. The doctest also shows the wrapping behavior of the implementation (for standard integers, shifting by more bits than the type has is UB (c) or panics (rust)).

Maybe the modulo behavior should be called out more explicitly still?

[folkertdev]

Looks like CI is using an older version of rustc? I can reproduce the failures with cargo test but they disappear with cargo +nightly test.