Consider deprecating `rotate_left`

[scottmcm]

Now that https://doc.rust-lang.org/std/primitive.slice.html#method.rotate_left is stable (1.26), consider deprecating the implementation here. Note that they're not identical; in the std version the shift length isn't modulo the array length. The std version is notably faster than this rotate implementation, however.

[bluss]

This whole crate is rusting! Thanks though