Fix: find_node() should not modify the avl, so make it a const parameter
Fix: avl_find() should not modify these parameters, so make them const
Fix: ht_find() should not modify these parameters, so make them const
Fix: The cfg should be constant when destroying the ll
Fix: Destroying an avl subtree should be done with constant cfg
Fix: Destroy avls with constant configuration
Fix: Destroying hashtable objects with constant configuration
Fix: ll_is_empty() shouldnt be able to alter the object it gets
Fix: ll_delete() should not be able to alter the cfg or the compare element
Merge pull request #191 from matthiasbeyer/fix-find_const_parameters

Fix: `_find()` functions shouldn't modify parameters, make them const
Merge pull request #193 from matthiasbeyer/fix-ll_is_empty_const_parameter

Fix: ll_is_empty() shouldnt be able to alter the object it gets
Merge pull request #192 from matthiasbeyer/fix-destroy_const_parameters

Fix: Destroying datastructures with const cfg
Fix: ll_ndel() should not be able to alter the cfg
Fix: Removing an element from the AVL should not alter the cfg or cmp data element
Fix: The avl_del() operation should not be able to alter the cfg or cmp data element
Fix: deleting elements by predicate should not alter cfg
Fix: avl_ndel() should not alter cfg
Fix: The ht_del() operation should not be able to alter cfg or cmp data element
Merge pull request #194 from matthiasbeyer/fix-delete_const_parameters

Fix: The `_del()` and `_ndel()` operations shouldn't be able to alter cfg or cmp element