Pull request description:
Enhance Witness struct element access methods:
- Rename `nth()` to `get()` for clearer slice-like element retrieval
- Introduce `get_back()` method for flexible reverse indexing
- Remove redundant `second_to_last()` and `third_to_last()` methods
- Add `#[track_caller]` to index implementation for better error tracking
- Update all references to use new method names
- Improve documentation with usage examples
The changes provide a more intuitive and consistent approach to accessing witness elements.
Close#4098
ACKs for top commit:
Kixunil:
ACK 3ca3218c23
tcharding:
ACK 3ca3218c23
apoelstra:
ACK 3ca3218c236c63a9b006047524e2b47e310f07d9; successfully ran local tests
Tree-SHA512: 163e7457f3fe5141373e27a6df5fe1da6f2f05f02e877ef96243510d030d832c0fa86ade781e015a3c392f004651170b60438a83d330f1059457e5ade6478af7
febce17eff