Fpath_sexp0.FsegmentSourcePart of a file path.
A Fsegment.t represents a segment of a file path, i.e., the parts of the path that are separated by the directory separator character.
For example, in the file path "/home/user/documents/file.txt", the segments are ["home" ; "user" ; "documents" ; "file.txt"].
A valid file segment cannot contain '/', the directory separator char or null characters.
By contrast to Fpath's seg, a Fsegment.t may not be empty.
This module provides functions to convert between strings and file segments, validate segments, and some common file segments.