This series of patches consists in

- the same 2 patches as v3
- Noam's refactoring
- some amendments ("occurrence", order of arguments in tests)
- a proof-of-concept for marking the occurrences with '^'

Here is a list of improvements that I plan on tackling Soonish™

1. refrain from adding markers if the minibuffer is not wide enough,

2. use (:inherit '(warning underline)) instead of warning, so that
    - if the warning face has some underlining, it is used,
    - otherwise the underline face makes sure that we don't rely only on
      colors.


Thank you all for your your reviews and your patience.  Sorry I can't
manage to take more time to work on this.