Can you add features like 'directory_iterator'?

[tangm421]

I also don't want to use bloated boost in some situations. your code provided a good choice. But can you add the directory_iterator similar boost? I think it's very useful at some times.

[tatsy]

I hope the commit by @tangm421 will be merged, which is really useful to avoid using Boost and to use filesystem in legacy C++ environments without C++17.

I also prepared directory_iterator which is more like the definition in STL.
tatsy@5783722
I can make a new PR if my implementation is more helpful.