Aggregate function combinators

A combinator is a suffix on the name of an aggregate function, as in sumIf. Combinators can be stacked, as in sumIfOrNull.

Suffix	Description
`State`	Returns the intermediate state of the aggregate as a value instead of the final result.
`Merge`	Combines intermediate states made by the State combinator and returns the final result.
`If`	Takes a condition as the last argument and applies the aggregate only to the rows where it holds.
`OrNull`	Returns NULL instead of the default value when the aggregate saw no rows.
`Array`	Takes arrays and applies the aggregate to every element as if each element were a row.
`ForEach`	Takes arrays and applies the aggregate to each position, returning an array.