Use result instead of the polymorphic equivalent

[cfcs]

It would be nice if vchan would use the result type rather than the equivalent polymorphic variants, for better composability with rresult continuations:

`Ok of 'a | `Error of string

lib/port.mli:

val of_string: string -> [ `Ok of t | `Error of string ]
(* replace with: *))
val of_string : string -> (t, [`Msg of string] ) result

lib/location.mli:

val of_order: int -> [ `Ok of t | `Error of string ]
(* replace with: *)
val of_order: int -> (t, [`Msg of string] ) result

lib/s.mli:

val port_of_string: string -> [ `Ok of port | `Error of string ]
(* replace with: *)
val port_of_string: string -> (port, [`Msg of string] ) result

[samoht]

That sounds like a very good idea. Patches are welcome :-)

[hannesm]

I submitted a PR addressing this issue as #130, happy to get it reviewed and merged!