Maybe remove Fpath.root, at least add Fpath.is_root.

[dbuenzli]

Fpath.root (which is Fpath.v "/") is certainly going to be wrongly used as stop condition when one loops up in a file hierarchy.

Formally the stop condition should be Fpath.(equal (parent p) p). But it seems better to add at least a Fpath.is_root function that checks if a path is a root directory, doing the right thing on paths with volumes (windows or //).