Attempt to fix SupplyOperation weirdness

When doing, for example, $s2 = $supply.grep(* == 1); $s2.tap(...);
$s2.tap(...); $supply.more(1); # would run both taps *twice*

See http://irclog.perlgeek.de/perl6/2014-07-09#i_8998922 and
http://irclog.perlgeek.de/perl6/2014-07-10#i_8998982 for camelia
examples.