Add support for BLAKE2b hashing algorithm

[suizman]

As @panchoh suggested it's worth trying BLAKE2b hashing algorithm. It suppose to be x2 faster than SHA256. We've find a Go implementation in BLAKE2b.

Adding support in QED of this algorithm should't be a difficult task. And performing some benchmark to compare between the SHA256 performance.

[panchoh]

I reckon that this is all we need to test it:

diff --git a/hashing/hash.go b/hashing/hash.go
index 1871e20..3a819ea 100644
--- a/hashing/hash.go
+++ b/hashing/hash.go
@@ -17,8 +17,9 @@
 package hashing

 import (
-   "crypto/sha256"
    "hash"
+
+   "golang.org/x/crypto/blake2b"
 )

 type Digest []byte
@@ -58,7 +59,7 @@ type Sha256Hasher struct {
 }

 func NewSha256Hasher() Hasher {
-   return &Sha256Hasher{underlying: sha256.New()}
+   return &Sha256Hasher{underlying: blake2b.New256(nil)}
 }

 func (s *Sha256Hasher) Salted(salt []byte, data ...[]byte) Digest {

[suizman]

Sure! Check-out the current state at this branch. I've also done some benchmarking with this results on my machine:

~/g/s/g/b/qed ⎇ blake2b ?
❯ go test -benchmem -short -run=\^\$ github.com/bbva/qed/hashing -bench \^\(Benchmark\)                                             
goos: linux
goarch: amd64
pkg: github.com/bbva/qed/hashing
BenchmarkSha256HasherRandomBytes-6       1000000              2712 ns/op             192 B/op          3 allocs/op
BenchmarkBlake2bHasherRandomBytes-6      1000000              1864 ns/op             192 B/op          3 allocs/op
BenchmarkSha256HasherFixedBytes-6       100000000              541 ns/op              64 B/op          2 allocs/op
BenchmarkBlake2bHasherFixedBytes-6      100000000              385 ns/op              64 B/op          2 allocs/op
BenchmarkBlake2bFixedBytesNoWrite-6     100000000              203 ns/op               0 B/op          0 allocs/op
BenchmarkSha256FixedBytesNoWrite-6      100000000              364 ns/op               0 B/op          0 allocs/op
PASS
ok      github.com/bbva/qed/hashing     154.161s

Just need to finish some tests before doing the PR