Feature/poseidon opt goff

[arnaucube]

Optimize Poseidon migrating from *big.Int to goff generated finite field operations.

Also fixes #4 and updates the Mimc7 usage of the CheckBigIntArrayInField, and updates BabyJubJub EDDSA to the new Poseidon methods.

Benchmarks: Tested on a Intel(R) Core(TM) i5-7200U CPU @ 2.50GHz, with 16GB of RAM.

• Before the optimizations:

  BenchmarkPoseidon-4              470       2489678 ns/op
  BenchmarkPoseidonLarge-4         476       2530568 ns/op

• With the optimizations of https://github.com/iden3/go-iden3-crypto/pull/12:

  BenchmarkPoseidon-4              766       1550013 ns/op
  BenchmarkPoseidonLarge-4         782       1547572 ns/op

• With the changes of this PR, where uses goff generated code instead of *big.Int:

  BenchmarkPoseidon-4             9638        121651 ns/op
  BenchmarkPoseidonLarge-4        9781        119921 ns/op

More than 10x improvement (from 1.547.572 ns/op to 119.921 ns/op).

Warning: do not merge until we check the license

[ed255]

The code looks good to me! Now the only remaining thing to merge this is resolve any doubt about licensing.

• All our code is published under GPLv3 and the copyright is held by 0kims (thus giving 0kims the possibility of distributing under a different license).
• Goff is Apache 2, which allows commercial distribution without disclosing the source code with the only requirements of:
  • Showing the Apache 2 copyright notice with the distribution
  • Stating the list of changes made to the code

So I think Apache 2 would cause any problem in either the GPLv3 licensed distribution of our libraries nor a possible commercial distribution under a private license (as long as the two requirements of the Apache 2 for Goff are met).

[arnaucube]

After internal confirmation that there are no issues with the license, will merge this PR to master.