Feed operator parallelism

feed.pm6

augment class Rakudo::Internals {
    method EVALUATE-FEED(Mu $input, @stages) {
        my Channel     $pipeline .= new;
        my Lock::Async $lock     .= new;

        $pipeline.send: $input;

        await @stages.map(-> &stage {
            start {
                $lock.protect({
                    my Mu $input  := $pipeline.receive;
                    my Mu $output := &stage($input);
                    $pipeline.send: $output;
                })
            }
        });

        my $output := $pipeline.receive;
        $pipeline.close;

        nqp::callmethod(
          $input,
          'STORE',
          nqp::callmethod($input, 'Slip'),
          nqp::callmethod($output, 'Slip')
        );
    }
}