v4: `path::compare` should separately compare root-name, root-directory and relative-path

[Lastique]

Currently, v3 and v4 perform path comparison element-wise, using iteration over path elements with iterators. This means that if one of the paths is missing root-name or root-directory, the algorithm may compare root-name or root-directory against a filename in the other path.

v4 should implement std::filesystem algorithm [fs.path.compare], which separately compares root-names, root-directories and then the elements of relative_path().