Add ungrouped aggregations; group before ordering

[zonotope]

This patch computes ungrouped queries with aggregation functions as if all results are in a single implicit group, supporting queries like:

'{:context  {:ex "http://example.org/ns/"}
  :select   [(count ?name)]
  :where    [[?s :schema/name ?name]]}

This patch also changes the grouping algorithm to avoid needing to sort the result set if there's no order-by clause specified.

This builds on #263, so take a look at that one first.

closes #239

[bplatz]

I think this looks really good. Not sure if you have a sense on why the test is failing.

Realize this is a draft, but would be great to get it committed since it touches highly volatile files at the moment, don't want to create a more challenging time getting it merged later (or vice versa for other merges).

[zonotope]

Not sure if you have a sense on why the test is failing.

This is a new (or, at least newly uncommented) test. It's failing in cljs, but I reflexively made it a cljc file when all the other tests are clj.

I've changed it to a clj file and will add an issue to make the tests we need work in cljs.