Move portray_clause/1 and listing/1 to their own module

[jgarte]

Hi,

I saw this comment here:

https://github.com/mthom/scryer-prolog/blob/master/src/lib/format.pl#L558

Should these predicates be moved to their own module?

[triska]

This could be worth considering at one point in the far future, when there are many more library predicates, with some of them maybe more related to a different (then arising) cluster than "formatting".

At present, I see no need to make any such changes, I think these predicates are sufficiently related to formatting to make library(format) a logical place to have them in, and I don't think that library(format) at present contains so many predicates that it is hard to keep in mind what it provides.

[triska]

Also, to clarify: The point is not to make a dedicated library for every predicate, but to recognize and use the best clustering.

As a simple example: In the future, there may be a set of predicates that deal with providing information about Prolog source code to editors. If such a set of predicates arises, and they form a meaningful cluster, say "Language Server Protocol", a hypothetical library(lsp), then listing/1 could maybe be a useful component of such a cluster, and even then moving a predicate should only happen after very careful consideration of trade-offs because existing code depends on the current library structure.