Do occur-check in unification instead of in subst-accumulation

[MxmUrw]

In #214 we took the easy way, but we should be able to have better error messages if we do this in unification itself, before adding the substitution.

Though this depends on how our error reporting is implemented.

[MxmUrw]

We need to evaluate how criticial it is to do this and add further error information. For this we need a test case which recreates this issue.

[MxmUrw]

Maybe we should pass a message when doing addSub, then it is fine to still do the check in substitution.

[MxmUrw]

As this is a pretty rare error, we do not add a message for now. This issue can become relevant again when there is an actual example where this error occurs.