Grokking Rust With Data Structure Implementations

These are exercises inspired by Gankro's book Learning Rust With Entirely Too Many Linked Lists and were done after looking away from the book for a few days.

mutable_linked_list takes a different approach to a safe implementation of the mutable singly-linked list that caused me much pain and learning when dealing with mutable iterators.
immutable_linked_list ended up being almost an exact replica.
mutable_doubly_linked_list uses owned next pointers and unsafe prev pointers but after a subtle oversight caught by the tests, it was refactored so that the unsafe reverse-traversing code has almost the identical structure as the safe forward-traversing code.

License

Distributed under the terms of both the MIT license and the Apache License (Version 2.0) license. See LICENSE-MIT and the LICENSE-APACHE for details.