Error in compilation

[Wkkkkk]

Hi, thanks for sharing this interesting project.

While trying it out, there was one error in instantiating last_type_impl as follows.

In file included from 
../yapp/test/test_pipeline.cpp:2:
../yapp/include/yap/pipeline.h:123:30: error: no member named 'type' in 'yap::detail::last_type_impl<std::tuple<>>'
  122 |               std::make_unique<typename detail::last_type_impl<
      |                                         ~~~~~~~~~~~~~~~~~~~~~~~
  123 |                   stages_t>::type::element_type>(std::forward<F>(fun)))))
      |                   ~~~~~~~~~~~^
/Users/kunwu/Workspace/playground/HelloCpp/HelloConcurrentCpp/yapp/include/yap/pipeline.h:303:12: note: in instantiation of function template specialization 'yap::Pipeline<>::Pipeline<(lambda at /Users/kunwu/Workspace/playground/HelloCpp/HelloConcurrentCpp/yapp/test/test_pipeline.cpp:47:21) &, void, unsigned int>' requested here
  303 |     return Pipeline(std::move(pl), std::forward<F>(transform));
 

It'd be very nice to hear from you and see if this could be fixed.