Stop Statement requiring a Claim

This fixes #264.

This basically reverts #188 and a lot of related breaking changes. I know I'm part of the problem because I did not commented enough on these changes and even tried to adapt code. But after running into more and more ugly, painful code (see #264) and thinking about this for days I came to the conclusion that this is the most sane way. Yes, the fact that Statement extends Claim is painful. But the current state of a Statement requiring a Claim is in no way better.

Ping @filbertkm.