Use std::filesystem instead of platform-specific directory traversal code

[jonstewart]

C++17 introduces the std::filesystem library, which allows for platform agnostic filesystem directory walking. It is disappointingly limited in its approach to nonportable metadata, but if all you need to do is walk a directory recursively, get the paths, and open the files, it works great and can rid your code of #ifdefs.

[jonstewart]

Now that I'm on the right branch, looks like you're well under way.

[simsong]

Yes. I am trying to put std::filesystem stuff everywhere.

[simsong]

I think that we're using it everywhere now.