Fsegment as a better name than Fpart

[mbarbin]

I noticed that in the Fpath library there's already a naming in place to designate the parts of the file path that are separated by the directory separator. The name in use is segment. I had missed it previously and ended up accidentally minting a new name for it : Fpart.

I am planning now on renaming Fpart -> Fsegment to make things more consistent.

Probably the easiest thing to do is to leave the older name for a release or two, marked as deprecated.

[mbarbin]

Implemented in #10