add Hash.const0 for trivial hash function that ignores its input

src/core/CCHash.ml

@@ -24,6 +24,7 @@ let combine4 a b c d =
 (** {2 Combinators} *)
 
 let const h _ = h
+let const0 _ = 0
 
 let int i = i land max_int
 let bool b = if b then 1 else 2

src/core/CCHash.mli

@@ -14,6 +14,12 @@ type 'a t = 'a -> hash
 val const : hash -> _ t
 (** [return h] hashes any value into [h]. Use with caution!. *)
 
+val const0 : _ t
+(** Always return 0. Useful for ignoring elements.
+    Example: [Hash.(pair string const0)] will map pairs [("a", 1)]
+    and [("a", 2)] to the same hash, but not the same as [("b", 1)]
+    @since NEXT_RELEASE *)
+
 val int : int t
 val bool : bool t
 val char : char t