Implement and fix list functions and their tests.

Convert list to doubly-linked list for easy traversal.

Use node pointers instead of duplicating the find function logic.

Use abstraction instead of using SDL dependencies.

Instead of using macros, use iterators built-in on each list for
filtering and traversing.

Remove symlinks and make codebase use common modules instead.

Allow current item pointer to have custom type.

Change the pool list initializations.

Add memory and data-related implementations for handling game data.