A couple of useful changes

 - Fix a bug with the linked list bookkeeping when using subscript.
   (last isn't updated at https://github.com/amitdev/lru-dict/blob/5c5be1248fed51fd4d3468aec8d18cf550df7d5c/lru.c#L153 )
   Given this is not the first bug of this kind, I added 2 helper methods that operate on the linked list. Nothing else operates on the list directly.
 - Properly INCREF the key when storing it in the node object.
 - Small refactoring - stop using X{INC,DEC}REF when arguments should be non-null, add asserts, kill OBJ_INCRFED which is non-portable.
 - Do the correct thing when overwriting a value as opposed to corrupting memory.
 - Slightly more efficient LRU_clear implementation.