How should tail call recursion be handled?

Inspired by recent advances in the R language, I went ahead and implemented a form of tail call recursion. However, it necessarily executes in a "non-standard" way, greedily evaluating arguments to recursive calls.

This means that expectations of R would apply to most calls, but might be unexpected for tail-recursive calls.

### Typical Evaluation

```r
f <- function(n, result) {
if (n > 0) {
cat(n, "\n")
f(n - 1, result)
} else {
result
}
}

f(3, stop("Done"))
#> 3
#> 2
#> 1
#> Error: Done
```

### Tail-Recursive Evaluation

```r
f <- function(n, result) {
if (n > 0) {
cat(n, "\n")
f(n - 1, result)
} else {
result
}
}

f(3, stop("Done"))
#> Error: Done
```

---

This is because `result` is necessarily greedily evaluated. Recursive calls will not work with promises dependent on the parent frame or non-standard evaluation. The "right" approach here is a bit unclear, so I'm going to wait to see how the R version feels with all of R's non-standard eval tricks to see if I can learn something from the design.