More aggregations

[jamescheney]

trafficstars

Links supports SQL's COUNT when the list length function is used in queries. Other aggregation functions could be supported the same way, e.g. SUM, AVERAGE, MAX, MIN. Together with deduplication, this would enable writing (flat) grouping and aggregation queries in Links, albeit not in a way that could result in idiomatic GROUP BY queries (since GROUP BY syntax is never generated by Links's SQL generator.)