trie: clean up iterator constructors

Make it so each iterator has exactly one public constructor:

- NodeIterators can be created through a method.
- Iterators can be created through NewIterator on any NodeIterator.

core/state/dump.go

@@ -22,6 +22,7 @@ import (
 
 	"github.com/ethereum/go-ethereum/common"
 	"github.com/ethereum/go-ethereum/rlp"
+	"github.com/ethereum/go-ethereum/trie"
 )
 
 type DumpAccount struct {
@@ -44,7 +45,7 @@ func (self *StateDB) RawDump() Dump {
 		Accounts: make(map[string]DumpAccount),
 	}
 
-	it := self.trie.Iterator()
+	it := trie.NewIterator(self.trie.NodeIterator())
 	for it.Next() {
 		addr := self.trie.GetKey(it.Key)
 		var data Account
@@ -61,7 +62,7 @@ func (self *StateDB) RawDump() Dump {
 			Code:     common.Bytes2Hex(obj.Code(self.db)),
 			Storage:  make(map[string]string),
 		}
-		storageIt := obj.getTrie(self.db).Iterator()
+		storageIt := trie.NewIterator(obj.getTrie(self.db).NodeIterator())
 		for storageIt.Next() {
 			account.Storage[common.Bytes2Hex(self.trie.GetKey(storageIt.Key))] = common.Bytes2Hex(storageIt.Value)
 		}

core/state/iterator.go

@@ -118,7 +118,7 @@ func (it *NodeIterator) step() error {
 	if err != nil {
 		return err
 	}
-	it.dataIt = trie.NewNodeIterator(dataTrie)
+	it.dataIt = dataTrie.NodeIterator()
 	if !it.dataIt.Next(true) {
 		it.dataIt = nil
 	}

core/state/statedb.go

@@ -481,7 +481,7 @@ func (db *StateDB) ForEachStorage(addr common.Address, cb func(key, value common
 		cb(h, value)
 	}
 
-	it := so.getTrie(db.db).Iterator()
+	it := trie.NewIterator(so.getTrie(db.db).NodeIterator())
 	for it.Next() {
 		// ignore cached values
 		key := common.BytesToHash(db.trie.GetKey(it.Key))