[bug] rb-tree: Calling the comparator on node values instead of keys

[donaldong]

https://github.com/fragglet/c-algorithms/blob/17cd712ef84948ce5c3faae911bc797ebad26e09/src/rb-tree.c#L377

This line should check the keys, not the values.

Separately, does this repro accept new features such as implementing the missing deletion functionality in rbtree?

Thanks!