Tracking issue for verifying the memory safety of the alloc::collections::btree::node module

[zhassan-aws]

This module is used in implementing the BTreeMap collection and it contains a lot of unsafe code:

https://github.com/rust-lang/rust/blob/fda509e817abeeecb5b76bc1de844f355675c81e/library/alloc/src/collections/btree/node.rs

Link to challenge: https://model-checking.github.io/verify-rust-std/challenges/0004-btree-node.html

EDIT: link to challenge PR: #26

[jaisnan]

Tracking issues should have links to the challenges they're tracking.

1. In case, they're already added to the book, I am linking them to the issues.
2. If not, can we add the links to the challenges to the issue description once the PR for that challenge has been merged?

[zhassan-aws]

Added a link to the challenge PR in the description.