Challenge 5: Verify functions iterating over inductive data type: `linked_list`

Link to PR: #30
Link to challenge: [0005-linked-list.md](https://github.com/model-checking/verify-rust-std/blob/main/doc/src/challenges/0005-linked-list.md)