refactor(furvm): remove executor handle

furdb/include/context.hpp

@@ -9,7 +9,7 @@
 struct context {
     furvm::context_p  context = std::make_shared<furvm::context>();
     furvm::mod_h      mod;
-    furvm::executor_h executor;
+    furvm::executor*  executor = nullptr;
     furvm::function_h mainFunction;
 
     bool running = true;

furdb/src/main.cpp

@@ -56,7 +56,7 @@ int main(int argc, char** argv) {
             std::cerr << "Failed to load module " << argv[1] << ": " << ex.what() << '\n';
             return 1;
         }
-        ctx.executor     = ctx.context->emplace_executor(ctx.context);
+        ctx.executor     = &ctx.context->allocate_executor();
         ctx.mainFunction = ctx.mod->function_at("main", furvm::function_sig{});
 
         ctx.mod->set_native_function("println", [](furvm::executor& executor) {