trie: remove dependency on ethdb

This removes the core/types -> leveldb dependency.

trie/trie.go

@@ -60,8 +60,12 @@ func init() {
 
 // Database must be implemented by backing stores for the trie.
 type Database interface {
+	DatabaseReader
 	DatabaseWriter
-	// Get returns the value for key from the database.
+}
+
+// DatabaseReader wraps the Get method of a backing store for the trie.
+type DatabaseReader interface {
 	Get(key []byte) (value []byte, err error)
 }