return / for parent / and basename /

[yomimono]

List.hd and List.tl throw Invalid_argument on empty lists. Instead
of allowing this, define / as its own parent and basename.

[yomimono]

I haven't kept track of fmt's API changes, but I think the CI failures are unrelated to this PR, as they all seem related to deprecated fmt functions.

[talex5]

This looks obviously correct to me. The ocamldoc in the interface file even specifies these behaviours explicitly!

[basename empty] is the empty string [""]
[parent empty] is [empty]