- Update PackSignY & UnpackSignY interface and description
- Add test for UnpackSignY & PackPoint

Add `add-2008-bbjlp` for point addition

Benchmarks (On a Intel(R) Core(TM) i7-8705G CPU @ 3.10GHz, with 32 GB of RAM):

```
- Old:
BenchmarkBabyjub/AddConst-8              1000000              1072 ns/op
BenchmarkBabyjub/AddRnd-8                  93417             12943 ns/op
BenchmarkBabyjub/MulRnd-8                    252           4797810 ns/op
BenchmarkBabyjub/Compress-8              7291580               166 ns/op
BenchmarkBabyjub/InCurve-8                611137              1999 ns/op
BenchmarkBabyjub/InSubGroup-8             615792              2021 ns/op
BenchmarkBabyjubEddsa/SignMimc7-8            126           9358542 ns/op
BenchmarkBabyjubEddsa/VerifyMimc7-8          124           9484005 ns/op
BenchmarkBabyjubEddsa/SignPoseidon-8                 126           9486484 ns/op
BenchmarkBabyjubEddsa/VerifyPoseidon-8               126           9622807 ns/op

- With new point addition algorithm:
BenchmarkBabyjub/AddConst-8              1356836               881 ns/op
BenchmarkBabyjub/AddRnd-8                 274112              4220 ns/op
BenchmarkBabyjub/MulRnd-8                    492           2474412 ns/op
BenchmarkBabyjub/Compress-8              6964855               197 ns/op
BenchmarkBabyjub/InCurve-8                608169              2008 ns/op
BenchmarkBabyjub/InSubGroup-8             618772              1954 ns/op
BenchmarkBabyjubEddsa/SignMimc7-8            238           4962397 ns/op
BenchmarkBabyjubEddsa/VerifyMimc7-8          235           5234883 ns/op
BenchmarkBabyjubEddsa/SignPoseidon-8                 240           5028720 ns/op
BenchmarkBabyjubEddsa/VerifyPoseidon-8               243           5226654 ns/op
```

Point Addition: ~3x
Point scalar Mul: ~1.9x
Signature (poseidon): ~1.88x
Verification (poseidon): ~1.84x

importing go-iden3