[sledge] Add Ord.( @? ) to lexicographically compose compare functions

Summary:
This allows easily defining lexicographic orders as a composition of
other orders. For example, the natural ordering on `int * string`
pairs can be written:
```
Ord.Infix.((Int.compare >|= fst) @? (String.compare >|= snd))
```

The `@?` name is chosen as a hybrid between `@@` which denotes
function composition and `<?>` which is the infix operator of
`Containers.Ord` for defining lexicographic orders.

Differential Revision: D29441161

fbshipit-source-id: ae9143cdc

sledge/nonstdlib/NS.mli

@@ -43,7 +43,17 @@ module Poly : sig
   val hash : 'a -> int
 end
 
-module Ord = Containers.Ord
+module Ord : sig
+  include module type of Containers.Ord
+
+  val ( @? ) : 'a t -> 'a t -> 'a t
+
+  module Infix : sig
+    include module type of Containers.Ord.Infix
+
+    val ( @? ) : 'a t -> 'a t -> 'a t
+  end
+end
 
 (** Function combinators *)
 

sledge/nonstdlib/NS0.ml

@@ -55,7 +55,17 @@ module Poly = struct
   let hash = Stdlib.Hashtbl.hash
 end
 
-module Ord = Containers.Ord
+module Ord = struct
+  include Containers.Ord
+
+  let ( @? ) c1 c2 x y = c1 x y <?> (c2, x, y)
+
+  module Infix = struct
+    include Infix
+
+    let ( @? ) = ( @? )
+  end
+end
 
 (** Function combinators *)