Hashing Structures helpers

[gakonst]

Depends on arkworks-rs/snark#104

It'd be nice to have a helper method as an extension trait over CanonicalSerialize which calls a cryptographically secure hash function (maybe make it take a Digest?) on the serialized input, and returns the fixed size hash of the provided data.

It'd be even better if it would also be available on structs composing such elements.

Example:

use blake2::Blake2b;
use generic_array::GenericArray;
use zexe::algebra::*;
use digest::Digest;

trait Hasher: CanonicalSerialize {
   fn hash<H: Digest>(&self) -> GenericArray<u8, H::OutputSize> {
       let buffer = self.serialize();
       let mut hasher = H::new();
       hasher.input(buffer);
       hasher.result()
   }
}

struct X<E: PairingEngine> {
    a: E::G1Affine,
    b: E::G2Affine,
    c: Vec<E::G1Affine>,
}

x = X::<E> {
  a: E::G1Affine::zero(),
  b: E::G2Affine::zero(),
  c: vec![],
}
let hash = x.hash::<Blake2b>();

This would serialize all elements (compressed or uncompressed? i don't think it matters) to a buffer, and then take the hash of the buffer.

It becomes a bit trickier I believe if the struct ends up having non-CanonicalSerialize elements.

e.g.

struct X<E: PairingEngine> {
    a: E::G1Affine,
    b: E::G2Affine,
    c: Vec<E::G1Affine>,
    transcript_hash: [u8; 64],
}