List.sum [] is now 0 (ocaml-batteries-team#916)

List.sum and List.fsum default to 0 (or 0.0) in case the list is empty.
Previously, this was raising Invalid_argument.

Closes ocaml-batteries-team#519

src/batArray.mlv

@@ -592,12 +592,13 @@ let min_max a =
     try ignore (min_max [||]); false with Invalid_argument _ -> true
 *)
 
-let sum = reduce (+)
-let fsum = reduce (+.)
+let sum = fold_left (+) 0
+let fsum = fold_left (+.) 0.
 
 (*$T sum
   sum [|1;2;3|] = 6
   sum [|0|] = 0
+  sum [||] = 0
 *) (*$T fsum
      fsum [|1.0;2.0;3.0|] = 6.0
      fsum [|0.0|] = 0.0

src/batList.mliv

@@ -298,12 +298,14 @@ val min : 'a list -> 'a
 
 val sum : int list -> int
 (** [sum l] returns the sum of the integers of [l].
-    @raise Invalid_argument on the empty list.
+    Returns [0] on the empty list.
+    Note: prior to NEXT_RELEASE, used to raise Invalid_argument on the empty list.
  *)
 
 val fsum : float list -> float
 (** [fsum l] returns the sum of the floats of [l].
-    @raise Invalid_argument on the empty list.
+    Returns [0.] on the empty list.
+    Note: prior to NEXT_RELEASE, used to raise Invalid_argument on the empty list.
  *)
 
 val favg : float list -> float

src/batList.mlv

@@ -1369,11 +1369,15 @@ let reduce f = function
 
 let min l = reduce Pervasives.min l
 let max l = reduce Pervasives.max l
-let sum l = reduce (+) l
+let sum l = fold_left (+) 0 l
+(*$= sum & ~printer:string_of_int
+  2 (sum [1;1])
+  0 (sum [])
+*)
 
 let fsum l =
   match l with
-  | [] -> invalid_arg "List.fsum: Empty List"
+  | [] -> 0.
   | x::xs ->
     let acc = ref x in
     let rem = ref xs in
@@ -1386,9 +1390,9 @@ let fsum l =
         rem := xs
     done;
     !acc
-(*$T fsum
-  try let _ = fsum [] in false with Invalid_argument _ -> true
-  fsum [1.;2.;3.] = 6.
+(*$= fsum & ~printer:string_of_float
+  0. (fsum [])
+  6. (fsum [1.;2.;3.])
 *)
 
 let favg l =