Make path::filename not return root directory

[Lastique]

This was originally reported in #88.

Unlike C++17 std::filesystem, path::filename returns root directory, if the path ends with the root directory. For example, path("/").filename() == "/", while C++17 mandates an empty filename in this case.

It is not clear why this was made this way, but it makes sense to change this behavior to make it closer to std::filesystem.