Add support for union and intersection types

[veewee]

Fixes #643

[theofidry]

Thanks a lot! I'm really be happy if that's the only change necessary for adding union support 😄

[veewee]

Thanks a lot! I'm really be happy if that's the only change necessary for adding union support 😄

Yeah me too! Looks like the system is set up in a very flexible way. Good job!
I also took the liberty of adding support for intersection types. Or would you rather have a separate PR for that once this one is done?

We already created a version of grumphp that uses the union types with box, that seems to work ok.

[theofidry]

Thank you @veewee