Use some more C++17-isms to clean up the code further

[baconpaul]

https://www.reedbeta.com/blog/python-like-enumerate-in-cpp17/ will help with all my vector loops for instance

I don't want to do this now but after XT it would be great to add a 'src/common/cpp_extensions' with some tricks like this that we use (or perhaps even make it a lib / submodule so i can use it in sc also one day).