[m-rev.] for review: move char_to_doc to pretty_printer.m

[Peter Wang]

On Tue, 20 Dec 2022 22:20:39 +1100 "Zoltan Somogyi" <zoltan.somogyi at runbox.com> wrote:
> 
> The attached diff moves the implementations of all the X_to_doc
> functions in the standard library to pretty_printer.m. The concept
> has been reviewed, but this diff contains much more than that, so I would
> like it reviewed. Would people be content to do so after commit,
> or should I wait for a review before then?
> 
> Zoltan.

> Move X_to_doc functions to pretty_printer.m.
> 

That's fine.

Peter