Composite constraint collectors

Constraint Streams currently have an upper limit on cardinality, specifically 4.

That means that the user can have at most 4 elements in their groupBy. If three of these elements are group keys, that leaves only 1 for a constraint collector. The user can easily work around this by providing a single tuple instead of 3 individual keys, but there may be an alternative solution.

A collector such as `ConstraintCollector.composite(avg(), count(), max(), min())` would allow the user to use 4 collectors in the space of one.