Bump std:: to C++17

Goal here is to get the project moving towards C++17.

Almost all modern tool-chains support C++17 to varying degrees. The most notable deviant is Android SDK but its std::filesystem support should be available soon(tm).

My previous attempts, years ago, was to replace boost::filesystem with std::filesystem... but we'll wait on that for awhile.

Now that we have all 4 major OSes under gitlab, we can see if a particular feature is supported or not.

Eventually I'd like to see a pipeline for FreeBSD as well.

https://docs.gitlab.com/runner/install/freebsd.html <-- not yet shared runner