add method Label.gist, .redo and .next

Label.gist looks like this (with colors):
perl6-m -e 'my $x; my $y; BAR: while $x++ < 2 { }; say BAR'
Label<BAR>(at -e:1, 'my $x; my $y; ⏏BAR:while $x++ < 2 { };')