Making diffs work again: sorting introductory paragraphs correctly

We now have 'introductory' paragraphs to Supplement I with labels that look something like this: [part, 'Interp', 'h1']. These need to be sorted correctly during diffs.

We update the outdated label sorting method (copying a similar one in -parser), ensure that introductory paragraphs come first in a sorting, and update and add tests.