Fix a bug I introduced in groupBy.
Start of HAVING support, with a ton of problems.

* non 1-to-1 JS operations wreak havoc,
* need to replace `Int` result of SelectorPhase with a list of paths through history,
* just filtering `$where` selector on `Mapping` isn’t good enough – EG, `count(*) > 10` is a Mapping … but it happens to contain a Reduction.
* don’t abuse the CannotObjectConcatExpr error
* won’t need the `len` param to `filter
* doesn’t unwind the group properly after the selector

Some queries that won’t work:
```sql
select city, sum(pop) from zips group by city having count(*) > 10
```
```sql
select city from zips group by city having count(*) > 10 and city like '%NEW%'
```

Also implemented math operations for JsExprPhase.