core/types: make Block.{ParentHash,SeedHash,MixDigest} []byte

There is no reason to keep them as ethutil.Bytes.

core/types/block.go

@@ -15,7 +15,7 @@ import (
 
 type Header struct {
 	// Hash to the previous block
-	ParentHash ethutil.Bytes
+	ParentHash []byte
 	// Uncles of this block
 	UncleHash []byte
 	// The coin base address
@@ -41,9 +41,9 @@ type Header struct {
 	// Extra data
 	Extra string
 	// SeedHash used for light client verification
-	SeedHash ethutil.Bytes
+	SeedHash []byte
 	// Mix digest for quick checking to prevent DOS
-	MixDigest ethutil.Bytes
+	MixDigest []byte
 	// Nonce
 	Nonce []byte
 }