split ast definitions into more files

Also move pathToString definitions into the new files

Change-Id: I08b92ed108392825ec87af1f2b8a49205265dd47
diff --git a/src/schema.hpp b/src/schema.hpp
index 487f820..73e89df 100644
--- a/src/schema.hpp
+++ b/src/schema.hpp
@@ -12,7 +12,7 @@
 #include <set>
 #include <stdexcept>
 #include <unordered_map>
-#include "ast.hpp"
+#include "ast_path.hpp"
 
 namespace yang {
 enum class ContainerTraits {