Adding #[must_use] mark to rotate function

[Boris00035]

Title says it all, this mark would make the rust compiler tell the user of the function that they should use the functions output, to signify that this function is not in-place (in contrast to flipv/fliph)

Personally caused me some confusion since i assumed that since flipv and fliph operate in-place, rotate would aswell, this flag would have made me realise that it doesnt earlier.