Add Doubly Linked List example to tutorial

[elaustell]

I have spent a long time working out this example (with lots of help from @bcpierce00 and @septract). This problem is quite difficult because there are multiple pointers to each element of a doubly linked list, and this makes reasoning about ownership nontrivial. I am hoping that it will be a helpful addition to the tutorial.

[elaustell]

I see that the CI check failed, but under details it says it is failing on linear search, which I didn't touch... Everything passes locally for me.

[septract]

Linear search is returning code 124, which is a timeout - I would guess this is unrelated to your changes, and just reflects the stuff the Cambridge folks are changing in CN on the master branch

[cp526]

Thanks very much!