v0.4.0

This release adds the ability to track external allocations (allocations
which are not stored in a Gc<T>) which also participate in pacing the garbage
collector. There is now a new (feature-gated) type allocator_api::MetricsAlloc
which implements the allocator-api2::Allocator trait which can be used to
automatically track the external allocation of collection types.

This release also adds (feature-gated) tracing support, which emits a span
per GC phase (propagate, sweep, drop), and events when collection resumes and
yields.

Release Highlights

• Tracked external allocations API which participates in GC pacing.
• Feature-gated support for a allocator_api2::Allocator implementation that
  automatically tracks allocation.
• Feature-gated support for hashbrown types, to automatically implement
  Collect on them.
• Feature-gated tracing support.
• Implement Collect for Box<T: ?Sized>.
• Add methods to project Write<Option<T>> and Write<Result<T, E>>.
• Don't fail to build by trying to implement Collect on Arc<T> for platforms
  without Arc.