[sledge] Add Option.or_else

Summary:
```
val or_else : (unit -> 'a option) -> 'a option -> 'a option
(** [or_else f] is [f ()] on [None] and otherwise identity *)
```

Reviewed By: jvillard

Differential Revision: D20637481

fbshipit-source-id: 72a9d930f

sledge/lib/import/option.ml

@@ -12,6 +12,7 @@ let pp fmt pp_elt fs = function
   | Some x -> Format.fprintf fs fmt pp_elt x
   | None -> ()
 
+let or_else ~f = function None -> f () | o -> o
 let cons xo xs = match xo with Some x -> x :: xs | None -> xs
 
 module Monad_syntax = struct

sledge/lib/import/option.mli

@@ -11,6 +11,9 @@ include module type of Core.Option
 val pp : ('a_pp -> 'a -> unit, unit) fmt -> 'a_pp -> 'a option pp
 (** Pretty-print an option. *)
 
+val or_else : f:(unit -> 'a option) -> 'a option -> 'a option
+(** [or_else ~f] is [f ()] on [None] and otherwise identity *)
+
 val cons : 'a t -> 'a list -> 'a list
 
 module Monad_syntax : Monad_syntax with type 'a t := 'a option