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

[Zoltan Somogyi]

For review by anyone.

Should we move the varset argument of write_variable
and related ops to *before* the var argument?
It would be consistent with the argument order
of the write_term ops, and more conducive to
folds over lists/sets of variables.

Zoltan.
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