Remove generics over the PRP

[arnauorriols]

While it's not a bad design per se to be generic over the Psuedorandom Permutation used within Spongos, when imagining a parallel universe where Streams is not plagued with the infamous F: PRP it's almost too tempting to get rid of it, on the grounds of simplicity and YAGNI.

Benefits of getting rid of PRP trait and assuming Keccak1600:

• The obvious one, we get rid of no less than 700 type parameters, with the PRP bound in the majority of them:
  • It is one of the biggest quick wins we can have in terms of simplification, and the improvement is massive
  
• We can get rid of a great deal of GenericArray
  • I've removed at lot of GenericArray in the upcoming PR, but PRP was a resisting feud.
• We can get rid of all the PhantomData of the application

Drawbacks:

• Users can no longer opt for a different PRP.
• the type parameter helped identify those places where the Spongos state is mutated. Without the type parameter, the API loses this expressive power.