Feature Request: Expose bytes publicly

[slodge]

We've seen the small helper method bytes in https://github.com/r-lib/httr/blob/cb4e20c9e0b38c0c020a8756db8db7a882288eaf/R/progress.R#L60 - it's a nice helper method to display a byte size.

Obviously it's not hard for us to copy, paste and MIT attribute this code, but (before we do) can I check whether there's any interest in adding bytes to the public API of httr? Happy to provide a roxygen block and maybe a few test cases as a PR

Thanks

Stuart