Nested modules Permutation and Hash + missing stubs for Hades

Still requires some thoughts. The idea is to have submodules:

• Bls12_381.Permutation.Hades implementing the Hades permutation
• Bls12_381.Permutation.Rescue implementing the Rescue permutation

To follow some implementations (Zcash in Orchard and Dusk)

• Bls12_381.Hash.Poseidon128: instantiation of Poseidon128 (note the index for the partial rounds is the first element)
• Bls12_381.Hash.Poseidon252: instantiation of Poseidon252 (note the index for the partial rounds if the last element)

I think about providing a fixed and variable length instance (for padding). The hash must be functorize. We can have something like

module Bls12_381.Hash.MakeFixedLengthPoseidon (S : Hades) (sig val length : int end) : sig 
  val hash : Fr.t array -> Fr.t
end

However, we instantiate a module for only one function. Which does not really make sense. And this functor must be duplicated. We can have instead:

module Hash : sig
  module MakePoseidon : sig  
    val fixed_length_hash : (module S : Hades) -> int -> (Fr.t array -> Fr.t)
    val variable_length_hash : (module S : Hades) -> (Fr.t array -> Fr.t)
  end 
end

We can have get a hash function with:

let hash inputs = Bls12_381.Hash.MakePoseidon.fixed_length_hash (module Hades128 : Bls12_381.Permutation.Hades) 3 inputs

For Poseidon128 and Poseidon252, we can have:

module Hash : sig
  module Poseidon128 : sig  
    val fixed_length_hash : int -> (Fr.t array -> Fr.t)
    val variable_length_hash : (Fr.t array -> Fr.t)
  end 
end

and the user chooses the hash function depending on the usecase:

let hash inputs = Bls12_381.Hash.Poseidon128.fixed_length_hash 3 inputs