[perl #117915] [BUG] X metaoperator fails with bag keys

[Christian Bartolomaeus via RT]

This works now (on Moar, Parrot and JVM):

$ perl6 -e 'my Bag $bag .= new: <foo foo bar>; for $bag.keys X $bag.keys -> $a, 
$b { say $a, $b }'
foofoo
foobar
barfoo
barbar

I added a test to S02-types/bag.t with the following commit: 
https://github.com/perl6/roast/commit/245c5988e7