Release v0.4.0

• Improved the ergonomics of weak pointers:
  • Removed Weakable and WeakableCc
  • Added Weak::new()
  • Renamed the weak-ptr Cargo feature to weak-ptrs
• Implemented common traits like Display, PartialEq, Eq, Hash, etc.
• Cleaned up the code and smaller improvements

Internal changes:

• Renamed CleanerFn to CleaningAction
• Renamed WeakMetadata to WeakCounterMarker
• A mutable borrow is now taken in the RefCell Trace implementation instead of an immutable borrow