+-------------------------------------------------+
| THREADED BINARY SEARCH TREE (.tbst) FILE FORMAT |
+-------------------------------------------------+
Every file in the .tbst format can be broken up into _nodes_. Every node is 24
bytes in total and looks like:

    1. A 4-byte integer key.
    2. A 4-byte integer value.
    3. A 4-byte "next" pointer.
    4. A 4-byte "prev" pointer.
    5. A 4-byte "left" pointer.
    6. A 4-byte "right" pointer.

The format encodes a threaded binary tree:
https://en.wikipedia.org/wiki/Threaded_binary_tree

Note the "pointer"s are byte indexes into the file where the corresponding node
starts.

In addition to the nodes, the .tbst file also has a 4-byte pointer at byte 0
that points to the first node.  A 0 pointer indicates pointing to NULL.
