Fix aggregations and extend set of built-in functions

- [x] COUNT is actually SUM at the moment, because I am an idiot
- [x] MIN and MAX do not group correctly
- [x] No support for something like Datomic's`:with` clause
- [ ] Potentially hairy type conversions
- [x] `Aggregation` might re-order relations, violating any underlying projection
- [ ] COUNT-DISTINCT
- [x] AVG
- [x] MEDIAN
- [x] VARIANCE
- [ ] STDDEV
- [x] Floating points

Datomic also offers a couple of things returning collections, some of which (e.g. `distinct`) directly map to Differential operators and should be treated as such.