* Switches to "positional notation" to fix the bug.
* Consolidates `encode_hint` and `generate_prefix_free_hint` into `encode_hint`.
* Moves prefix_free_hints.go to hints_generator.go; `encode_hint` and `decode_hint` were moved here too.
* The "prefix-free" logic was made 0-index based instead of 1-index based for two reasons:
1. To simplify 1-off discrepancies.
2. To keep backwards compatibility with the old usage of `encode_hint`.
* Slight perfomance improvement added by reusing the runt-to-index map instead of rebuilding for each call to `decode_hint`.
* Unit tests added; for all new logic and old affected logic.
Fixes#10231
