To complement #826, implement the situations where a SELECT subquery is coerced into an array rather than a singleton.

It is worthwhile to verify w.r.t. SQL what is the correct semantics here (coerce to the first row vs to the first column).

This will entail defining a built-in function for computing the coercion, as well as naming it. This could also be a good opportunity to weigh what would be good names for this function as well as for COLL_TO_SCALAR -- to best reflect their semantics as well as highlight their similarity. This aspect overlaps with #42.