Defer keyword

R has the `on.exit` construction for delaying execution of code, but I feel like this capability should be more central to a language. Taking inspiration from the `zig` language, providing a more native `defer` syntax would help to keep cleanup more localized.

To also take a nod from `withr` and `rust`'s lifetimes, I think `defer` should also act as a function that accepts a frame, allowing a function to defer execution until a specific frame completes execution. This is a bit like `python`'s context managers, but with the added benefit that they can be attached to arbitrary scopes and do not rely on implementing object-specific entry and exit methods.

```r
n_lines <- function(file) {
f <- open(file)
defer close(f)

length(read_lines(f))
}
```

```r
connect <- function(file) {
f <- open(file)

defer (parent_frame(-1)) {
close(f)
}

f
}
```