v0.6.0

Big release with a huge number of changes.

Much faster DynamicRootSet that does not require individual allocations per
root!

Non-object-safe Collect trait enables more optimization for automatic impls.

Explicit forward and backwards GC write barriers to enable more kinds of safe
container abstractions.

More precise control over GC phase.

Entirely new, much more tunable, much more correct GC pacing system.

Removes the idea of "external allocation" from collection entirely, as it was
simply too complex to reason about. Instead, users can use custom allocators
and directly adjust collection debt up and down based on recorded external
allocation. This direct adjustment of collection debt can also be use to
directly drive incremental collection independent of GC allocation.

Release Highlights

• Implement Ord/PartialOrd for CollectionPhase.
• Let Gc::cast, Gc::from_ptr GcWeak::cast and GcWeak::from_ptr work with
  unsized types.
• Redesign DynamicRootSet to not require individual allocations per root.
• Remove Collect, Sized, and 'static bounds on Rootable::Root.
• Add convenience Gc::new_static, Gc::erase and GcWeak::erase convenience
  methods.
• Make StaticCollect<T> work with unsized T.
• Add (unsafe) forward write barriers and explicit (unsafe) backwards write
  barriers to Mutation.
• Rename CollectionPhase::Collecting -> CollectionPhase::Sweeping and
  StaticCollect -> Static.
• Get rid of unsafe_empty_collect! macro.
• Slightly re-org crate exports and drop some special case re-exports from the
  crate root.
• Rename MarkedArena::start_collecting to MarkedArena::start_sweeping to
  match CollectionPhase::Sweeping
• Implement Borrow<T> for Gc<'gc, T>.
• Rework the Collect trait. Adds a 'gc lifetime for correctness.
  Collect::NEEDS_TRACE associated constant allows for more optimization
  of tracing. No longer object safe! Adds a Trace trait for the trace
  implementation passed to Collect::trace.
• Use unsafe interior mutability to reduce internal RefCell locking in the
  collector.
• Update hashbrown to 0.17.
• New, much more correct GC pacing system with many more tunable parameters.
• Add Arena::cycle_debt (equivalent of Arena::collect_debt which stops at
  the end of the GC cycle. and some method renames to aid phase tracking.
• Add the ability to add / subtract artificial debt from the GC pacing.
• Improvements to correctness around inheriting GC debt across cycles.
• Implement Collect for BinaryHeap.
• Add OnceLock (a GC-aware OnceCell).
• Add {Deref, Index}Write traits, for deref. and indexing behind write
  barriers.
• Migrate to Edition 2024 to fix field! macro unsoundness.
• Add a DynCollect trait and a way to safely GC custom dyn traits via a macro
  (replacement for lost Collect object safety).
• Add Collect impl for enum_map::EnumMap behind feature gate.
• Add Collect impl for slotmap::SlotMap behind feature gate.
• Add Collect impl for smallvec::SmallVec behind feature gate.
• Collect impl for indexmap::IndexMap and indexmap::IndexSet behind
  feature gate.
• Remove all notions of externally allocated bytes, MetricsAlloc, and
  allocator-api2 dependency.