Calling `.tail()` on a empty slice panics

Calling `.tail()` on a slice does no check at all (see https://github.com/rust-lang/rust/blob/master/src/libcore/slice.rs#L210 ), and will panic if called on a empty slice.

Nothing suggests it in the documentation, I personally expected that calling `.tail()` on an empty slice would return an empty slice, as `.first()` returns an Option.
