Supply.zip should eager-shift its values

Otherwise we're at the mercy of the thing that runs out downstream
emit in order to have correct behavior. Shaken out by refactors to
Supply concurrency control.

src/core/Supply.pm

@@ -1170,7 +1170,7 @@ my class Supply does Awaitable {
                 else {
                     whenever $supply -> \val {
                         @values[$index].push(val);
-                        emit( $(@values.map(*.shift).list) ) if all(@values);
+                        emit( $(@values.map(*.shift).list.eager) ) if all(@values);
                     }
                 }
             }