Introduce nanopass pre-/postconditions

[masak]

To-do before merging: rebase on a merged #383. Done.

Closes #368.

Work to be done:

• [x] Add preconditions
• [x] Add postconditions
• [ ] Provide a simpler/more convenient/more extensible and changeable way to specify tree pattern matching
• [ ] Make sure the error messages are excellent

[masak]

Note to self: have two different ways to flag up broken pre/postconditions: one for temporary stuff and one for permanent stuff. That way, the (excellent) error messages can indicate this and provide better situational guidance.