Refactor ListInstance to a header file

This typedef is useful for the upcoming `move` command, so I want to
reuse it. It made no sense to put it in ast_values.hpp, so I just made a
new header.

Change-Id: Ia1ad07b8ac393a3f1aeffcdd4e19776390a00907
diff --git a/src/datastore_access.hpp b/src/datastore_access.hpp
index 6412037..f9ea901 100644
--- a/src/datastore_access.hpp
+++ b/src/datastore_access.hpp
@@ -12,6 +12,7 @@
 #include <optional>
 #include <string>
 #include "ast_values.hpp"
+#include "list_instance.hpp"
 
 /*! \class DatastoreAccess
  *     \brief Abstract class for accessing a datastore
@@ -36,8 +37,6 @@
 
 class Schema;
 
-using ListInstance = std::map<std::string, leaf_data_>;
-
 class DatastoreAccess {
 public:
     using Tree = std::map<std::string, leaf_data_>;