split ast.hpp into more files

Change-Id: I534437662aaec87e706e2e88c3cd8daaa951a6e9
diff --git a/src/ast.cpp b/src/ast.cpp
index 87f464d..9b1210c 100644
--- a/src/ast.cpp
+++ b/src/ast.cpp
@@ -6,6 +6,7 @@
  *
 */
 #include "ast.hpp"
+#include "ast_handlers.hpp"
 
 InvalidKeyException::~InvalidKeyException() = default;