Consider design change in light of GHC join point work

The [join points paper](https://www.microsoft.com/en-us/research/publication/compiling-without-continuations/) suggests that it may now be possible to avoid the need for `Skip` altogether. It would be worth checking if that's actually the case. If so, it might be possible to simplify the implementation considerably. Furthermore, I seem to recall that the presence of skips (which allow vectors of the same length to be represented by streams of different lengths) prevents one or more optimizations. I vaguely remember trying to do something a bit clever with `replicate` and finding it impossible because of skips, but I don't remember any details.