Add scanner/valuer interface to babyjub.SignatureComp

- Add `add-2008-bbjlp` for point addition
- Add `goff` to BabyJubJub point addition

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

- Old (commit: e04ca5764a):
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 (commit: aab1a681dd):
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

- With new point addition algorithm & goff (current commit):
BenchmarkBabyjub/AddConst-8              3000531               400 ns/op
BenchmarkBabyjub/AddRnd-8                2770335               428 ns/op
BenchmarkBabyjub/MulRnd-8                   6636            175522 ns/op
BenchmarkBabyjub/Compress-8              7358768               180 ns/op
BenchmarkBabyjub/InCurve-8                539193              1950 ns/op
BenchmarkBabyjub/InSubGroup-8             601402              1958 ns/op
BenchmarkBabyjubEddsa/SignMimc7-8           2940            409487 ns/op
BenchmarkBabyjubEddsa/VerifyMimc7-8         2908            414407 ns/op
BenchmarkBabyjubEddsa/SignPoseidon-8                2395            493165 ns/op
BenchmarkBabyjubEddsa/VerifyPoseidon-8              2491            494849 ns/op

Point Addition: ~9.86x
Point scalar Mul: ~14x
Signature (poseidon): ~10.2x
Verification (poseidon): ~10.56x

---

Total improvement (from old to current):
Point Addition: ~30.24x
Point scalar Mul: ~27.33x
Signature (poseidon): ~19.24x
Verification (poseidon): ~19.44x
```

babyjub/eddsa.go

@@ -177,6 +177,24 @@ func (s *SignatureComp) Decompress() (*Signature, error) {
 	return new(Signature).Decompress(*s)
 }
 
+// Scan implements Scanner for database/sql.
+func (s *SignatureComp) Scan(src interface{}) error {
+	srcB, ok := src.([]byte)
+	if !ok {
+		return fmt.Errorf("can't scan %T into Signature", src)
+	}
+	if len(srcB) != 64 {
+		return fmt.Errorf("can't scan []byte of len %d into Signature, want %d", len(srcB), 64)
+	}
+	copy(s[:], srcB[:])
+	return nil
+}
+
+// Value implements valuer for database/sql.
+func (s SignatureComp) Value() (driver.Value, error) {
+	return s[:], nil
+}
+
 // Scan implements Scanner for database/sql.
 func (s *Signature) Scan(src interface{}) error {
 	srcB, ok := src.([]byte)

babyjub/eddsa_test.go

@@ -135,6 +135,17 @@ func TestCompressDecompress(t *testing.T) {
 	}
 }
 
+func TestSignatureCompScannerValuer(t *testing.T) {
+	privK := NewRandPrivKey()
+	var value driver.Valuer //nolint:gosimple this is done to ensure interface compability
+	value = privK.SignPoseidon(big.NewInt(674238462)).Compress()
+	scan := privK.SignPoseidon(big.NewInt(1)).Compress()
+	fromDB, err := value.Value()
+	assert.Nil(t, err)
+	assert.Nil(t, scan.Scan(fromDB))
+	assert.Equal(t, value, scan)
+}
+
 func TestSignatureScannerValuer(t *testing.T) {
 	privK := NewRandPrivKey()
 	var value driver.Valuer
@@ -142,8 +153,8 @@ func TestSignatureScannerValuer(t *testing.T) {
 	value = privK.SignPoseidon(big.NewInt(674238462))
 	scan = privK.SignPoseidon(big.NewInt(1))
 	fromDB, err := value.Value()
-	assert.NoError(t, err)
+	assert.Nil(t, err)
-	assert.NoError(t, scan.Scan(fromDB))
+	assert.Nil(t, scan.Scan(fromDB))
 	assert.Equal(t, value, scan)
 }
 
@@ -157,8 +168,8 @@ func TestPubKeyScannerValuer(t *testing.T) {
 	value = pubKValue
 	scan = pubKScan
 	fromDB, err := value.Value()
-	assert.NoError(t, err)
+	assert.Nil(t, err)
-	assert.NoError(t, scan.Scan(fromDB))
+	assert.Nil(t, scan.Scan(fromDB))
 	assert.Equal(t, value, scan)
 }
 

go.sum

@@ -126,6 +126,7 @@ github.com/steakknife/bloomfilter v0.0.0-20180922174646-6819c0d2a570 h1:gIlAHnH1
 github.com/steakknife/bloomfilter v0.0.0-20180922174646-6819c0d2a570/go.mod h1:8OR4w3TdeIHIh1g6EMY5p0gVNOovcWC+1vpc7naMuAw=
 github.com/steakknife/hamming v0.0.0-20180906055917-c99c65617cd3 h1:njlZPzLwU639dk2kqnCPPv+wNjq7Xb6EfUxe/oX0/NM=
 github.com/steakknife/hamming v0.0.0-20180906055917-c99c65617cd3/go.mod h1:hpGUWaI9xL8pRQCTXQgocU38Qw1g0Us7n5PxxTwTCYU=
+github.com/stretchr/objx v0.1.0 h1:4G4v2dO3VZwixGIRoQ5Lfboy6nUhCyYzaqnIAPPhYs4=
 github.com/stretchr/objx v0.1.0/go.mod h1:HFkY916IF+rwdDfMAkV7OtwuqBVzrE8GR6GFx+wExME=
 github.com/stretchr/testify v1.2.2/go.mod h1:a8OnRcib4nhh0OaRAV+Yts87kKdq0PP7pXfy6kDkUVs=
 github.com/stretchr/testify v1.3.0 h1:TivCn/peBQ7UY8ooIcPgZFpTNSz0Q2U6UrFlUfqbe0Q=