Adopt scalar type hints and return types

[pfrenssen]

Now that we no longer support PHP 5 we can adopt scalar type hints and return types for our interfaces and protected methods.

Since changing interface signatures is a B/C break we should do this before we release a beta version.

I don't think it is necessary to perform this task in a single PR because of our large number of interfaces. It's OK to spread this over a number of PRs that cover a few interfaces/classes each. That would make it easy to review + merge.