There is a TODO in the `StringWithSubj` method because the value
types really depend on the subject type (for example, `count` uses
uint64, while `sum` uses int64). I'm waiting to address this
until we decide how to handle sums of floats (Decimal), because
that will affect this logic as well.
This PR adds support for a `having` argument in a `GroupBy` query.
Usage looks like this:
```
GroupBy(Rows(a), having=Condition(count > 10))
GroupBy(Rows(a), aggregate=Sum(field=b), having=Condition(sum > 100))
```
