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.

I don't understand the question.

The symbols `cons` and `prepend` are references to the **same** function.

`prependTo` is not the same as `cons` or `prepend` (or `flip` for that matter.)  Granted the implementation of `prependTo` makes reference to the functions referred to by `flip` and `prepend`, it is identical to neither.  Unless you think it also deserves a second name, it is not an alias to anything.