Replace Vec-compatible wrappers with clean collection accessor methods

- Remove complex CompressedKeyView/ValueView wrapper types (3 structs + implementations)
- Add direct collection accessor methods that hide internal structure:

  Key accessors:
  - get_key(index), first_key(), last_key()
  - keys_iter(), collect_keys()

  Value accessors:
  - get_value(index), get_value_mut(index)
  - first_value(), last_value(), first_value_mut(), last_value_mut()
  - values_iter(), values_iter_mut(), collect_values()

  Pair accessors:
  - get_pair(index), get_pair_mut(index)
  - first_pair(), last_pair(), first_pair_mut(), last_pair_mut()
  - collect_pairs()

- Add custom ValuesMutIter for safe mutable iteration over values
- Use raw pointers to solve borrowing conflicts in pair_mut methods
- Add comprehensive tests (55 total tests, all passing)
- Add demonstration of external code usage patterns
- Maintain clean encapsulation - no internal structure exposure
- Provide compatibility methods (collect_*) for existing code migration

Benefits:
- Cleaner API: No complex wrapper types to understand
- Better encapsulation: Internal structure completely hidden
- Flexible: Easy to add new accessors without breaking changes
- Efficient: Direct access without wrapper overhead
- Familiar: Similar to standard Rust collection APIs

Co-authored-by: Ona <no-reply@ona.com>

Author: KentBeck