dict CHANGE split dictionary API to private and public part

Make part of the dictionary API public and usable by library users.
It should allow more effective way of memory usage.
diff --git a/src/xml.c b/src/xml.c
index c1abf37..6f8a7c8 100644
--- a/src/xml.c
+++ b/src/xml.c
@@ -28,7 +28,7 @@
 #include <unistd.h>
 
 #include "common.h"
-#include "dict.h"
+#include "dict_private.h"
 #include "printer.h"
 #include "tree_schema.h"
 #include "xml_private.h"