Function process

example/use-with-bthread/anyflow_multi_nodes.cpp:11–14

struct AddProcessor : public GraphProcessor {
  virtual int process() noexcept override {
    *c.emit() = *a + *b;
    return 0;
  }

  ANYFLOW_INTERFACE(ANYFLOW_DEPEND_DATA(int32_t, a, 0)
                        ANYFLOW_DEPEND_DATA(int32_t, b, 1)