trie: remove dependency on package crypto

Package crypto needs cgo, which is inconvenient for some build
configurations.

trie/trie.go

@@ -22,7 +22,7 @@ import (
 	"fmt"
 
 	"github.com/ethereum/go-ethereum/common"
-	"github.com/ethereum/go-ethereum/crypto"
+	"github.com/ethereum/go-ethereum/crypto/sha3"
 	"github.com/ethereum/go-ethereum/logger"
 	"github.com/ethereum/go-ethereum/logger/glog"
 )
@@ -30,11 +30,14 @@ import (
 var (
 	// This is the known root hash of an empty trie.
 	emptyRoot = common.HexToHash("56e81f171bcc55a6ff8345e692c0f86e5b48e01b996cadc001622fb5e363b421")
-
 	// This is the known hash of an empty state trie entry.
-	emptyState = crypto.Keccak256Hash(nil)
+	emptyState common.Hash
 )
 
+func init() {
+	sha3.NewKeccak256().Sum(emptyState[:0])
+}
+
 // Database must be implemented by backing stores for the trie.
 type Database interface {
 	DatabaseWriter