Make IntegerRangeType represent open intervals properly

[jlherren]

This removes the usage of PHP_INT_MIN and PHP_INT_MAX to mark open-ended intervals and instead uses null for that purpose. Since using getMin() and getMax() now became slightly more complicated for callers, a lot of IntegerRangeType-related logic has been moved into the class itself.

I especially like that some type combinator logic is now inside the type class, but without too much pressure, since it is allowed to return null when it can't handle the situation. Perhaps this could be extended into the entire Type hierarchy.

[ondrejmirtes]

I really like this approach, moving stuff to Type usually tends to clean things up :) I'd definitely like to have the method on Type, but it should have a more specific name, like shiftInteger or something. It would allow also handling for UnionType of IntegerRangeTypes for example. And it should return ErrorType in cases where it doesn't make sense (shifting object by an integer amount for example).

I'll review and merge this PR as it is, we can move and rename the method in a late rPR.

[ondrejmirtes]

Or shiftByInteger?

[ondrejmirtes]

I feel like method names should be verbs (commands)...

[jlherren]

Is this okay now, or should I change anything?

[ondrejmirtes]

Yes, thank you very much :) As a next step, we should move shift() to Type. I think it could be used for PostInc and similar operations not only on IntegerRangeType (and unions of them), but also on ConstantIntegerType in a meaningful way.