[m-rev.] for post-commit review: add remaining X_to_string and format_X ops

[Julien Fischer]

On Tue, 4 Jul 2023, Zoltan Somogyi wrote:

> Should we move the varset argument of write_variable
> and related ops to *before* the var argument?

IMO, yes.

> It would be consistent with the argument order
> of the write_term ops, and more conducive to
> folds over lists/sets of variables.

The diff looks fine.

Julien.