Remove the conclusion type parameter of InferenceJustifier

[ykazakov]

I do not see much use for the conclusion type parameter of InferenceJustifier.
Since there is no type parameter for type of inferences, a justifier should work with arbitrary inferences (over given conclusions). But any meaningful implementation of this interface would need to check the type of inferences (over concrete conclusions), and the conclusion type parameter will not be really used. Two possible fixes I see:

• Either use a generic inference parameter <I extends Inference<?>>:
  J getJustification(I inference);
• Or do not use any input parameter:
  J getJustification(Inference<?> inference);

[ykazakov]

Alternatively on can use guava's Functor instead of Justifier

[ykazakov]

Alternatively on can use guava's Functor instead of Justifier