feat(CCInt64): add popcount operation

adapted from CCInt, but directly on int64, so it works for
Int64.{min_int/max_int}.

src/core/CCInt64.ml

@@ -11,6 +11,30 @@ let hash x = Stdlib.abs (to_int x)
 
 let sign i = compare i zero
 
+(* see {!CCInt.popcount} for more details *)
+let[@inline] popcount (b:t) : int =
+  let m1 = 0x5555555555555555L in
+  let m2 = 0x3333333333333333L in
+  let m4 = 0x0f0f0f0f0f0f0f0fL in
+
+  let b = sub b (logand (shift_right_logical b 1) m1) in
+  let b = add (logand b m2) (logand (shift_right_logical b 2) m2) in
+  let b = logand (add b (shift_right_logical b 4)) m4 in
+  let b = add b (shift_right_logical b 8) in
+  let b = add b (shift_right_logical b 16) in
+  let b = add b (shift_right_logical b 32) in
+  let b = logand b 0x7fL in
+  to_int b
+
+(*$= & ~printer:CCInt.to_string
+  0 (popcount 0L)
+  1 (popcount 1L)
+  63 (popcount max_int)
+  1 (popcount min_int)
+  10 (popcount 0b1110010110110001010L)
+  5 (popcount 0b1101110000000000L)
+*)
+
 let pow a b =
   let rec aux acc = function
     | 1L -> acc

src/core/CCInt64.mli

@@ -30,6 +30,10 @@ val hash : t -> int
 (** [hash x] computes the hash of [x].
     Like {!Stdlib.abs (to_int x)}. *)
 
+val popcount : t -> int
+(** Number of bits set to 1.
+    @since NEXT_RELEASE *)
+
 val sign : t -> int
 (** [sign x] return [0] if [x = 0], [-1] if [x < 0] and [1] if [x > 0].
     Same as [compare x zero].