Fix memory leaks

include/syntax/langs.h

@@ -1,6 +1,7 @@
 #ifndef SYNTAX_LANGS_H
 #define SYNTAX_LANGS_H
 
+#include "scripting/decl.h"
 #include "syntax/decl.h"
 
 #define DEF_LANG(name)                                                         \
@@ -15,6 +16,12 @@
     #name, { name##_parse, name##_state_match }                                \
   }
 
+struct CustomState {
+  mrb_value state;
+  CustomState(mrb_value s) : state(s) { mrb_gc_register(mrb, state); }
+  ~CustomState() { mrb_gc_unregister(mrb, state); }
+};
+
 template <typename T>
 inline std::shared_ptr<T> ensure_state(std::shared_ptr<T> state) {
   using U = typename T::full_state_type;