Factor out binary tree and linked list helpers

[domfarolino]

For a lot of algorithms that use linked lists and binary trees, I often repeat the same struct for each respectively, set up an example, and then create the same helpers (usually print and delete or something). These helpers and struct definitions get repeated a lot, so it might make sense to factor them out into utilities, or even use the homemade data structures in this repository (though for some problems that might not be possible, given problem-specific modifications).