Simplify with C++17 #46

Saving note from an email, there are a ton of a simplifications that can be made now that C++17 is required:

A lot of code needs better comments, and there are also big swathes that are overcomplicated because they were written to use c++11, and can now be simplified with c++17. Examples:

• AUTO_RETURN macro uses can be replaced with auto return types
• Priority<0>, Prioriity<1>, Priority<2> overloads used for conditional compilation can all go away and be replaced with if constexpr
• std::tuple indexing code can go away and be replaced with std::apply

Basically all of mp/util.h can go away. Also kj::mvCapture uses can replaced with shorter direct captures.

I guess all of this was fixed already, except for Priority? Maybe the issue can be updated?

There are also some trivial/minimal fixups:

diff --git a/include/mp/util.h b/include/mp/util.h
index 3074201..ffbb51b 100644
--- a/include/mp/util.h
+++ b/include/mp/util.h
@@ -101,7 +100,7 @@ struct Split<index, TypeList<Type, _Second...>, TypeList<_First...>, false>
 template <typename Callable>
-using ResultOf = decltype(std::declval<Callable>()());
+using ResultOf = std::invoke_result_t<Callable>;
 
-//! Substitutue for std::remove_cvref_t
+//! Type helper removing cv-qualifiers and references.
 template <typename T>
-using RemoveCvRef = std::remove_cv_t<std::remove_reference_t<T>>;
+using RemoveCvRef = std::remove_cvref_t<T>;
 

Another thing would be CTADs, but this requires bumping the min compiler versions ...

Yes a lot of the cleanups mentioned above are taken care of now. I think the biggest remaining improvement might be modernizing the clientInvoke / serverInvoke code which was written for C++11 and has a pretty archaic and convoluted style. I suspect there are a lot of simplifications that can be made there.