I'd like to use parser{,_gophermap,_gemtext}.c in standalone testing programs, and this function is getting in the way since it depends on ui.c

Add a parser_serialize function and an optional function `serialize' for parsers. The default implementation just prints the text line, which is suitable for parser_textplain and parser_textpatch; gemtext and gophermap uses their own specific unparse functions. This is intended for an incoming cache refactoring: we'll cache the page serialized in the correct format, not once it has been translated to gemtext. This will allow in the future to provide commands such as ``save page to disk'' or interface with an on-disk cache.