Level up the passing of the caller environment across R <--> C++ boundary

[jennybc]

vroom has several cases where R calls C++, which calls back into R again. And we may need the original caller env in order to schedule cleanup or mention the right function in an error. We can do a better job of this. jennybc/parentframecpp is a toy package where I really characterized the problem. The solution will look something like jennybc/parentframecpp#1, which might just mean using/inlining cleancall.

[jennybc]

Good food for thought re: this waves hands whole situation from @DavisVaughan's review of #599:

I think if I were implementing this, knowing how annoyingly complex this is, I think I would pass through both call and another env called env_cleanup or env_defer. Even if it is the same environment as call, it feels like it would express the meaning better

When I see call, I'm conditioned to think "this has to do with error messaging", so seeing it used for other things confuses me and smells off to me.

I'd also be happy if some other approach nukes the need for this entirely, but I also wouldn't be mad if this were the final solution, where vroom() itself just has this at the very top:

vroom <- function(...) {
  # The frame we tie all temporary file cleanup to
  env_cleanup <- environment()
}

Another thing I'd probaby do is make env_cleanup a required argument to each helper, rather than setting to caller_env(), because that is never going to be the correct environment and would lull me into a false sense of correctness