Assert pre/post/invariants of phases

It would be great if each [`Phase`](https://github.com/effekt-lang/effekt/blob/master/effekt/shared/src/main/scala/effekt/Phase.scala) would have an additional method(s) that could check the pre- and postconditions and thus maintain invariants of the ASTs.

There should be a `debug` flag or similar in the [config](https://github.com/effekt-lang/effekt/blob/master/effekt/jvm/src/main/scala/effekt/EffektConfig.scala) that enables this. Since this might slow down the compilation and CI significantly we might only want to run this when merging into master and not on each push.

Ideally, we would have some reusable components to check invariants such as `NoLocalDefinition`, `NoAdditionalFreeVars`, `TypeChecks`, ...