Proposed New sort API

[David Bremner]

I started looking at William's sorting patches [1], but the
proliferation of sorting options bugged me a bit. I decided to sketch
out a new more flexible API.

In the new API, there is is a sort "key", currently mapped one-to-one
to value slots, but potentially could do more sophisticated things
like making keys up dynamically (e.g. preprocessing subjects).

The second parameter is a sort "type". Currently this is just
ascending or descending, but other potential options include
sort_by_relevance_then_value

[1]: id:20170926053547.18564-1-jb55 at jb55.com