Use Pratt's parser for expressions in text blocks

This one is ready for review for which I would be really grateful. The diff might look big but at the end of the day there are rather simple code changes and plenty of test changes.
Oh, thnx, good catch! Actually the comment is wrong - we've discussed this use-case with @b-laporte in the morning and decided to change the existing semantic. But yes, this comment needs to be updated.