Implement ordering by aggregates

Right now, it's not possible to order by aggregates, so that queries like

[:find
 (count ?x)
 :where
 [_ :mentions ?x]
 :order
 (count ?x)]

are not possible. (They don't parse, due to :order expecting bare variables.)

@rnewman suggests this is simply not implemented, and says: the obvious solution is to take each :order that’s a valid aggregate expression and push it into the projector, just as we do now with order-vars. It would perhaps be worthwhile to make sure that

[:find ?person (sum ?balance)
 :where …
 :order (desc (sum ?balance))]

generates

SELECT datoms00.e AS ?person, sum(datoms00.v) AS ?balance … ORDER BY ?balance DESC

rather than mentioning sum twice. Otherwise this should mostly be a matter of extending the parser and altering some type choices in the projector/ordering hookup, which already exists.