Fix: Warn on unused linked list function call return values
Fix: check return value of ll_insert()
Fix: Check return value of ll_delete()
Fix: Warn on unused result for avl function calls
Fix: Warn on unused result for internal avl function calls
Fix: Warn on unused result for hashtable function calls
Fix: Check return value of ht_init() in tests
Merge pull request #242 from matthiasbeyer/add-warn_unused_result

Add __warn_unused_result__