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.

Sorry, I should have been clearer. I'm talking about the variable `prepend` as an :alien: for `R.prepend`. My question is when do we decide to `var foo = R.foo;` and use `foo` elsewhere versus using `R.foo` directly? The answer is not obvious since we *sometimes* refer to `R.foo` directly (as with `R.cons = R.prepend;`).