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.
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