Use single State associated type in item spec

[azriel91]

Originally, logical and physical states were separated on the basis that implementors must distinguish the two concepts, and diff only happens on the logical state.

However, it is becoming apparent that:

• The separation of logical and physical state does exist, but certain things only have a logical state, and requiring State<Logical, Physical> everywhere is unnecessary complexity.
• Consistency of a single state type per item spec has the advantage of:
  • Not needing to go through the state.logical layer in each *Spec function
  • Serialization is consistent (always the same type)
  • Serialization is simplified (no additional layer as well)
• Implementors may still use State<Logical, Physical> when it makes sense for the item. We should make it easily discoverable in the documentation.

This should be done early since the effort that grows is large per new functionality added to the framework.