Add Legacy.Result for OCaml >= 4.8.0 (#913)

Now, users can pattern match Result.t returned from some library using
Legacy.Result.Error while Batteries is opened.

Ref #839

src/batteries.mlv

@@ -47,6 +47,7 @@ module Legacy = struct
   module Big_int = Big_int
   module Bigarray = Bigarray
   module Str = Str
+##V>=4.8## module Result = Result
 end
 
 (* stdlib modules *)