Fix: ll_delete() should not be able to alter the cfg or the compare element
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
Added implementation for ht_ndel()
Fix: ht_ndel() should not be able to alter the passed cfg