performance of a zip in for's list.

for-a-z-b perf

$ time perl6 -e 'my @a = (^64); my @b = (^64); my $i = 0; while ($i++ < 1000) { for @a Z @b -> $a, $b { } }'

real	0m11.031s
user	0m10.992s
sys	0m0.032s
$ time perl6 -e 'my @a = (^64); my @b = (^64); my @c = @a Z @b; my $i = 0; while ($i++ < 1000) { for @c -> $a, $b { } }'

real	0m0.900s
user	0m0.876s
sys	0m0.020s
$ time perl6 -e 'constant @a = (^64); constant @b = (^64); my $i = 0; while ($i++ < 1000) { for @a Z @b -> $a, $b { } }'

real	0m10.813s
user	0m10.788s
sys	0m0.020s