split ast definitions into more files

Also move pathToString definitions into the new files

Change-Id: I08b92ed108392825ec87af1f2b8a49205265dd47
diff --git a/tests/cd.cpp b/tests/cd.cpp
index 2cef615..8fe30ad 100644
--- a/tests/cd.cpp
+++ b/tests/cd.cpp
@@ -7,7 +7,7 @@
 */
 
 #include "trompeloeil_catch.h"
-#include "ast.hpp"
+#include "ast_commands.hpp"
 #include "parser.hpp"
 #include "schema.hpp"