Remove the Arena allocator dependency from @linkedlist

The idea is that we can change the `generic_linked_list.jinja2` template to make the arena allocator optional. So we can implement a linkedlist that uses `malloc`, or inject the custom allocator we want

Start by reading the generic_linked_list.jinja2 template and tracing how the Arena allocator is currently used for @linkedlist. Verify the generated linkedlist can use malloc and can accept an injected custom allocator, then check the existing linkedlist generation or usage paths for regressions.

Written by the indexing model from the issue text.