Fix scoping error due to the introduction of `Result.compare` in result.1.5.

[mjambon]

Hi, we ran into this error:

# Error: The implementation src/zed_string.ml
# [...]
#          val compare_index :
#            t ->
#            ('a, 'b) result ->
#            ('a, 'b) result ->
#            ok:('a -> 'a -> int) -> error:('b -> 'b -> int) -> int
#        is not included in
#          val compare_index : t -> index -> index -> int
#        File "src/zed_string.mli", line 184, characters 0-46:
#          Expected declaration
#        File "src/zed_string.ml", line 454, characters 6-19:
#          Actual declaration

The strange signature for the compare function is from the Result module which provides its own compare since version 1.5.

Hopefully this PR fixes the build for all OCaml versions supported by zed. See also the opam-repository PR https://github.com/ocaml/opam-repository/pull/15909

[kandu]

Many thanks, @mjambon :)