ref: add type/classname result for each gen_* function

src/compiler/generator/gen_handle.c

@@ -3,11 +3,11 @@
 #include <stdbool.h>
 
 // Opaque pointer
-bool gen_handle(fzl_gen_t *gen, fzl_node_t *node)
+gen_res_t gen_handle(fzl_gen_t *gen, fzl_node_t *node)
 {
   if (!gen || !node)
-    return false;
+    return (gen_res_t){.error = true};
 
   type_buf.kind = FZL_EXTERN_KIND_HANDLE;
-  return true;
+  return (gen_res_t){.error = false};
 }