Fix Sequence.Reverse.rest

Without this fix, `[1].reversed.rest` is `[1]` even though it should be `[]`, because the `Sequence.Reverse.rest` implementation fails in the case that `size==1` (⇒ `size-2 == -1` ⇒ `outer[size-2..0] == outer[-1..0]` ⇒ `rest == outer[0..0]`).