Clean up parsing & pretty-printing.
  
  Most list fields now have consistent syntax:
  
    - commas are optional
    - each element of the list is either
  	- a sequence of one or more non-space non-comma characters
  	- a quoted string in Haskell 98 lexical syntax
  
  The build-depends field still requires commas, because the elements
  can contain whitespace.
  
  I merged fieldGet/fieldShow in StanzaField into a single field for
  simplicity.