add R.appendTo and R.prependTo

These functions are certainly useful: I use `R.append` and `R.prepend` in conjunction with `R.flip` or `R.rPartial` quite often. They're also easy to name. These two facts make them a straightforward addition.

For consistency, perhaps we should add `R.concatTo` as well? I find myself flipping `R.concat` from time to time as well.