Expose orderClause from PersistQuery.hs

[ivb-supercede]

This is part of the followup to #1298.

This exposes orderClause in a similar manner to filterClause, to help third-party libraries which want to format their own SQL queries using Persistent's internal helpers.

It rewrites orderClause a bit to bring it in line with the other exposed functions, but there should be no functional changes as a result.

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• Update the Changelog.md file with a link to your PR
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[parsonsmatt]

looks good, thanks!