Creating a progressor object outside `with_progress()`?

So I've deprecated `.progress` in furrr, and was planning on writing a vignette on using progressr.

My first thought was to create an example like the one below, but it errors. It seems like the `progressor()` call _must_ be called from within `with_progress()`. Is that right? I tried to look around for documentation about this but I can't find anything, maybe I'm just missing it.

I know there is a developer/user separation of the APIs, but as of right now I think that this is how users might want to construct progress enabled functions to pass to `future_map()`. Does this seem reasonable?

```r
library(progressr)
#> Warning: package 'progressr' was built under R version 4.0.2
library(purrr)

set.seed(123)

x <- replicate(n = 10, runif(20), simplify = FALSE)
p <- progressor(steps = length(x))

slow_fn <- function(x) {
p()
Sys.sleep(.1)
sum(x)
}

with_progress({
map(x, slow_fn)
})
#> Error in error("length(timestamp) == 0L") :
#> .validate_internal_state(‘handler(type=update) ... end’): length(timestamp) == 0L
#> Error in error("length(timestamp) == 0L") :
#> .validate_internal_state(‘reporter_args() ... begin’): length(timestamp) == 0L
```