til::rle - Add debug checks to containers and iterators

# Description of the new feature/enhancement

The following would be "nice to haves" for `til::rle`:
* [ ] debug assertions to ensure no zero-length runs are inserted (this would break `til::rle::scanner`)
* [ ] checked iterators

Start by locating the implementation of til::rle and its scanner, then inspect how runs are inserted and how iterators are exposed. The work is complete when zero-length runs are rejected in debug checks and iterator misuse is checked, with the relevant tests updated or added if the project has coverage for these containers.

Written by the indexing model from the issue text.