Rework Pod6 Parser

Due to the slowness of the first attempt at backlink placing, we're
going this route in hope of something faster. This is just the changes
to Syngen::Pod6. The necessary changes in syngen.p6 are coming up.
Finish fragment placing.

Note that some things may be broken, most notably placing backlinks
whose section was not found.

Takes around 3800s on my machine, hopefully it'll be faster elsewhere
and in the future.