Refactor block_to_rd()

[hadley]

Most tags can be processed in order, so we could loop over the list of tags once, rather than multiple times. There are three exceptions:

• @backref has default if not supplied
• @usage has default if not supplied
• @aliases NULL prevents default from being added

This could be powered by S3 (by adding a subclass in RoxyTag. It will considerably simplify the code, make it easier to add new tags, and (I suspect) improve performance.

[hadley]

@param, @slot and @field (which call process_def_tag()) currently create a single named list, but it feels like that could be handled in at the topic level.

[hadley]

I just moved backref into block_find_object(); it's possible @usage and @aliases should go there too.