Assemble sort keys into an array, and pick them out, each with correct orderType

Fixes many cases, including the example given in #107.

Many other queries currently trip up the planner, including when filtering is present; see the pending tests included in this PR.
With the beta approaching quickly, and then prod release, the time for FIXMEs and TODOs has officially ended. :)

I'm not checking in any more pending work (unless I modify pending work and reduce the amount that is pending).