Abstract progress bars

I think it might be possible to turn bits of code like:

```r
if (verbose) {
pb_warmup <- create_progress_bar(
phase = "warmup",
iter = c(self$warmup, n_samples),
pb_update = pb_update,
width = self$pb_width
)

iterate_progress_bar(
pb = pb_warmup,
it = 0,
rejects = 0,
chains = self$n_chains,
file = self$pb_file
)
} else {
```

into something like

```r
pb_warmup <- create_progress_bar_warmup()
iterate_progress_bar(pb_warmup, 0, 0)
```

Perhaps by putting a couple of objects into the slots of R6 objects? It's a small change, but it happens in a couple of places and it might just be another small thing to improve clarity when viewing the code