Arrays: make type enum public

[ljacqu]

In ArrayUtils most code is based on determining the array component type of the Object and then doing something with it—this can actually be useful for outside callers, too.

• Make the array type enum public
• Currently it holds the component type as reference type, which is a bit confusing. How to deal with that?
• Probably should have a method to get an Optional<ArrayType> and one where it just throws