create IsInState interface, add val for it to InStateSideEffectBuilder

[gabrielittner]

We'll need to split state checks into 2 different kinds:

• One for checking whether we should start this side effect, this will match the current checks
• The other one for whether we should terminate the side effect. For the current things this will be the same but for anything inside the identity block this will need to check that the identity of the current state is still the same as the identity of the state that started the side effect. The reason for separating isInState for this purpose is to avoid reimpleting all side effects again for the identity block.
  To avoid mistakes I want to have 2 separate interfaces for these 2. This introduces the first one that just matches the current checks. The second one will be introduced further into the refactoring.

InStateSideEffectBuilder will have this as an abstract val which already allows using it directly in runOnlyIfInInputState and will enable the next pr to share a bit more code for the side effects.