Improve Rakudo's compliance with S26

This pull request improves Rakudo's compliance with S26 (POD and documentation), namely in the area of declarative comments.  Rakudo currently supports `#=` as a leading comment; however, it should be treated as a trailing one, and `#|` should be treated as a leading comment.  This pull request implements that.

There's a lot of work yet to be done on this feature, but I finally got my simple tests to pass, so I thought this would be a good time to put it up for review!