feat: add prepared error/null ok returns from gen_* functions

src/compiler/generator/gen_reference.c

@@ -9,16 +9,16 @@
 gen_res_t gen_reference(fzl_gen_t *gen, fzl_node_t *node)
 {
   if (!gen || !node)
-    return (gen_res_t){.error = true};
+    return gen_error;
 
   char *refname = (char *)node->token->value;
   ssize_t slot  = gen_resolve_extern_type(gen, refname);
   if (slot == -1)
-    return (gen_res_t){.error = true};
+    return gen_error;
 
   type_buf.kind = FZL_EXTERN_KIND_REF;
   type_buf.ref_target =
       (fzl_extern_type_ref_t){.is_primitive = false, .type_slot = slot};
 
-  return (gen_res_t){.error = false, .type = FZL_TYPE_NULL, .classname = NULL};
+  return gen_ok_null;
 }