metadata: store comments in lists since lists are used everywhere else

metadata: store comments in lists since lists are used everywhere else

The only data produced by parsing metadata that was stored as a tuple was
the comments' key/values.  Everywhere else, lists are used.  This changes
the comments to also only use lists to keep the data consistent internally.
That makes it vastly easier to produce the exact same metadata dict when
parsing .txt, JSON, XML, etc.