Re-organization of the classes

[alexcrt]

As the library is growing, maybe we could create some packages to have a cleaner structure. I was thinking of something like this:

Any thoughts?

[poetix]

I think the Spliterators should remain package-private, as there's no reason to expose them directly. Therefore they would have to be in the same package as StreamUtils. No objection to pulling out selector and function.

[alexcrt]

This is indeed a good point. I think there's no rush at the moment, but I guess it's good in the future to think about some structure and to avoid the all-in-one package.

[poetix]

I've pulled out selectors, functions and maps.