Simplify with C++17

[ryanofsky]

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.