Fix "Seq has already been iterated" sinking a spent Seq

The sink-all is only there to actually run lazily executed code when the
results are sunk. If the Seq has already been iterated we don't have to
bother.

src/core/Seq.pm

@@ -102,7 +102,7 @@ my class Seq is Cool does Iterable does PositionalBindFailover {
     }
 
     method sink() {
-        self.iterator.sink-all;
+        self.iterator.sink-all if $!iter.DEFINITE;
         Nil
     }