Proposed refactoring of the logics (no instances)

- Logic schemes (like ILogic or BILogic) are in their
  own file with minimal dependencies.
  - These files include the core definition + self-contained
    auxiliary lemmas.
- Separate file for instances (see Logics.Prop example).
  - Interesting question will be whether this will pollute
    some files because if you only care about the ILogic
    then you shouldn't include the BILogic instances as well.