Ellipsis collection into a variable

In R, ellipsis are often collected into a list of values using `function(...) { list(...) }`, but if we are to add #47, then we don't really have a good way of collecting "the rest". Instead, we should have some way of collecting ellipsis contents into a variable.

```r
f <- function(a, b, ..rest) {
}
```

This would assign a new variable `rest` which contains the contents of `list(...)`.

Together with #47, this would mean that we could also do something like:

```r
list(head, ..tail) <- list(1, 2, 3, 4, 5, 6, 7)
```