Add Int.sum

This method takes an Iter and sums its values, meaning that instead of
this:

    vals.into_iter.reduce(0, fn (acc, val) { acc + val })

You can write this instead:

    Int.sum(vals.into_iter)

Changelog: added

std/src/std/int.inko

@@ -4,7 +4,7 @@ import std.cmp (Compare, Equal, Ordering)
 import std.float (ToFloat)
 import std.fmt (Format as FormatTrait, Formatter)
 import std.hash (Hash, Hasher)
-import std.iter (Peekable)
+import std.iter (Iter, Peekable)
 import std.ops (
   Add, BitAnd, BitOr, BitXor, Divide, Modulo, Multiply, Power, ShiftLeft,
   ShiftRight, Subtract, UnsignedShiftRight,
@@ -194,6 +194,15 @@ class builtin Int {
     }
   }
 
+  # Sums the values of `iterator` into a single `Int`.
+  #
+  # # Examples
+  #
+  #     Int.sum([10, 20, 30].into_iter) => 60
+  fn pub static sum[I: Iter[Int]](iterator: move I) -> Int {
+    iterator.reduce(0, fn (acc, val) { acc + val })
+  }
+
   # Formats `self` as a `String` in the given format.
   #
   # # Examples

std/test/std/test_int.inko

@@ -385,4 +385,6 @@ fn pub tests(t: mut Tests) {
     t.equal(2.checked_pow(2), Option.Some(4))
     t.equal(MAX.checked_pow(2), Option.None)
   })
+
+  t.test('Int.sum', fn (t) { t.equal(Int.sum([10, 20, 30].into_iter), 60) })
 }