v0.5.0

This release adds the concept of "finalization" to arenas. At the end of the
"mark" phase, at the moment that an arena is considered "fully marked" and
is ready to transition to the "sweep" phase, the "fully marked" arena can be
examined with some special powers...

MarkedArena::finalize can be called to check which pointers are considered
"dead" for this collection cycle. A "dead" pointer is one that is destined
(without further changes) to be freed during the next "sweep" phase. You can
"resurrect" these pointers either through normal root mutation before entering
the "sweep" phase, or more simply by calling explicit resurrect methods
on those pointers. Any resurrection, whether through mutation or resurrect
methods, immediately makes the arena no longer fully marked, but the arena can
then be re-marked and re-examined.

This simple, safe API (examining a "fully marked" arena for potentially dead
pointers, performing mutation, then potentially re-marking and re-examining)
is enough to implement quite complex garbage collector behavior. It can be used
to implement "finalizer" methods on objects, "ephemeron tables", Java-style
"reference queues" and more.

Release Highlights

• New Finalization API.
• New API to query the current phase of the collector.
• Fixup the pacing algorithm for correctness and simplicity. Makes
  Pacing::with_timing_factor(0.0) actually work correctly for stop-the-world
  collection.
• GcWeak::is_dropped now works only during collection, not mutation.
• Add more trait impls to StaticCollect.
• Gc::ptr_eq and GcWeak::ptr_eq now ignore metadata of dyn pointers,
  matching the behavior of Rc::ptr_eq and Arc::ptr_eq.