add Poseidon&MiMC7 test vectors, Poseidon multiHash&hashBuffer, MiMC7 hashBuffer

[arnaucube]

This PR adds some test vectors for Poseidon&MiMC7 and some functions in order to make the library more usable from external repositories and to ensure compatibility with other implementations (such as go and rust):

• Poseidon

  • poseidon.hash: exposes generic Poseidon hash function with nRoundsF=8, nRoundsP=57, t=6
  • poseidon.multiHash: splits the bigint array into chunks of five elements and performs the Poseidon hash over chunks of five elements
  • poseidon.hashBuffer: performs Poseidon hash over a buffer array, first splitting the bytes into 31 bytes bigints, and then performing the multiHash
  • added test vectors in test/poseidon.js

• MiMC7

  • mimc7.hashBuffer: performs MiMC7 hash over a buffer array, first splitting the bytes into 31 bytes bigints, and then performing the multiHash
  • added test vectors in test/mimc7.js

Question about the multiHash:

• in src/mimc7.js (line 55) the multiHash does the cycles and at the end of each cycle does:

r = F.add(
            F.add(
                r,
                arr[i]
            ),
            exports.hash(bigInt(arr[i]), r)
        );

Why is doing r = ( r + arr[i] ) + hash(arr[i], r) instead of r = r + hash(arr[i], r)?
(https://eprint.iacr.org/2016/492.pdf page 5)

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