ArrayBuilder: array type should reflect common class of its entries

WHAT:

The ArrayBuilder (used on syntax like [1,3,5]) tries hard to unbox if possible (Integer to int) or keep Number as common class but falls short of strong typing for other classes.

It would be convenient to type arrays using the strongest (in sense of abstraction) common class from the entries.

HOW:

When detecting entry class mismatch (and not a unboxing or Number), find better common class; Object class should only be last resort.