Adds Fpath.sexp_of_t and defines 3 new modules: Fsegment, Absolute_path and Relative_path

[Fpath_sexplib0] is an OCaml module designed to be opened to extend the fpath package. It introduces three new modules to the scope: [Fsegment], [Absolute_path] and [Relative_path].

[Absolute_path] and [Relative_path] are helper modules that distinguish between classes of paths in the type system, enhancing type safety for applications manipulating paths.

Fpath is shadowed and retains all its original functionality, with the addition of a sexp serializer and new helpers for casting between the types of paths offered by the package (absolute and relative paths).