crypto: add BLAKE2b and BLAKE2s hash constants

Fixes golang/go#19060
Change-Id: I1844edc3dcccc8d83a11d1145b60b2b92f2658ca
Reviewed-on: https://go-review.googlesource.com/36876Reviewed-by: Adam Langley <agl@golang.org>

src/crypto/crypto.go

@@ -36,6 +36,10 @@ const (
 	SHA3_512                    // import golang.org/x/crypto/sha3
 	SHA512_224                  // import crypto/sha512
 	SHA512_256                  // import crypto/sha512
+	BLAKE2s_256                 // import golang.org/x/crypto/blake2s
+	BLAKE2b_256                 // import golang.org/x/crypto/blake2b
+	BLAKE2b_384                 // import golang.org/x/crypto/blake2b
+	BLAKE2b_512                 // import golang.org/x/crypto/blake2b
 	maxHash
 )
 
@@ -55,6 +59,10 @@ var digestSizes = []uint8{
 	SHA3_512:    64,
 	MD5SHA1:     36,
 	RIPEMD160:   20,
+	BLAKE2s_256: 32,
+	BLAKE2b_256: 32,
+	BLAKE2b_384: 48,
+	BLAKE2b_512: 64,
 }
 
 // Size returns the length, in bytes, of a digest resulting from the given hash