Refactoring MDS component

[zhenfeizhang]

Summary

refactoring MDS component in Poseidon so it can be used by Rescue

Proposal

• move mds sampling out from Poseidon module
• use the method https://github.com/EspressoSystems/Marvellous/blob/master/instance_generator.sage#L265 as the default MDS matrix. e.g., for q = 21888242871839275222246405745257275088548364400416034343698204186575808495617, we have z = 5, that is, we can use the echelon form of the following matrix :

[              1               1               1               1               1               1               1               1]
[              1               5              25             125             625            3125           15625           78125]
[              1              25             625           15625          390625         9765625       244140625      6103515625]
[              1             125           15625         1953125       244140625     30517578125   3814697265625 476837158203125]

• a separate API to sample MDS from rng via the method in Poseidon paper, i.e., https://github.com/arkworks-rs/sponge/blob/d20d9afce98e42dc9bd02950231afc5234e60b0c/src/poseidon/traits.rs#L131

Concretely:

pub(crate) struct MDS{
  ... 
}

impl Default for MDS {
  // use the rescue method
}

impl MDS {
 fn sample<R: Rng>(rng: &mut R)->Self{
   // use poseidon method
   // also need to decide if we want to do those three checks
 }
}

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