Supply.batch: handle no args and $elems == 1

In most cases, the returned supply from the batch method emits arrays of
batched elements.  There's no reason why the following should happen:

$s.batch(elems => 1, seconds => 1000)
  tap sees arrays
$s.batch(elems => 1)
  tap doesn't see arrays

I also think that just batch() should do something sane and also return
a supply that produces arrays.  This provides us a very consistent
interface from the tap-side where we know post-batch we should always be
expecting an array of values.

src/core/Supply.pm

@@ -850,7 +850,6 @@ my class Supply does Awaitable {
     }
 
     method batch(Supply:D $self: :$elems, :$seconds ) {
-        return self if (!$elems or $elems == 1) and !$seconds;  # nothing to do
         supply {
             my @batched;
             my $last_time;
@@ -895,7 +894,7 @@ my class Supply does Awaitable {
             else { # just $elems
                 whenever self -> \val {
                     @batched.push: val;
-                    flush if @batched.elems == $elems;
+                    flush if !$elems or @batched.elems == $elems;
                     LAST { final-flush; }
                 }
             }