Drop CKSUM and SRCTIME params; add as attributes

`@书社:cksum`, `@书社:mtime`, and `@书社:identifier` are now all added
during the parsing phase. (`@书社:identifier` used to be added during
the transformation phase, but badly. `@书社:mtime` is new.)

This hardcodes the location of the metadata file for now; ideally the
metadata would be embedded.