Use `mrb_bool` as return type of `mrb_const_defined_at`.

include/mruby/variable.h

@@ -48,7 +48,7 @@ void mrb_iv_set(mrb_state *mrb, mrb_value obj, mrb_sym sym, mrb_value v);
 mrb_bool mrb_iv_defined(mrb_state*, mrb_value, mrb_sym);
 mrb_value mrb_iv_remove(mrb_state *mrb, mrb_value obj, mrb_sym sym);
 void mrb_iv_copy(mrb_state *mrb, mrb_value dst, mrb_value src);
-int mrb_const_defined_at(mrb_state *mrb, struct RClass *klass, mrb_sym id);
+mrb_bool mrb_const_defined_at(mrb_state *mrb, struct RClass *klass, mrb_sym id);
 mrb_value mrb_mod_constants(mrb_state *mrb, mrb_value mod);
 mrb_value mrb_f_global_variables(mrb_state *mrb, mrb_value self);
 mrb_value mrb_gv_get(mrb_state *mrb, mrb_sym sym);

src/variable.c

@@ -1085,7 +1085,7 @@ retry:
   return FALSE;
 }
 
-int
+mrb_bool
 mrb_const_defined_at(mrb_state *mrb, struct RClass *klass, mrb_sym id)
 {
   return mrb_const_defined_0(mrb, klass, id, TRUE, FALSE);