0.3

An enormous number of breaking API changes, too many to list, almost the entire
API has been rethought.

The credit goes mostly to others for the release, @Bale001, @Aaron1011,
@dragazo, and especially @moulins.

Release Highlights

• New Arena API that does not require macros and instead uses a Rootable
  trait and HRTBs for lifetime projection.
• Methods on Arena to directly mutate the root type and map the root from one
  type to another.
• A new API for 'static roots that are held in smart pointers
  (DynamicRootSet).
• Gc pointers can now point to DSTs, and there is an unsize! macro for
  unsizing coercions to replace the unstable Unsize trait.
• Weak pointers!
• GcCell has been replaced by explicit public lock types held within Gc
  pointers with safe ways of mutating them.
• Field projection on held lock types to allow for separate locks held within a
  single Gc pointer to be safely mutated.
• Unsafe Gc pointer coercions to compatible pointee types.
• Soundly get references to Gc types with &'gc lifetime.
• More ergonomic Mutation and Collection context types.
• Tons of correctness and soundness fixes.

This release also completely drops the gc-sequence combinator crate.
Because of other API changes, anything you could do with gc-sequence before
can almost certainly be expressed better either using the new 'static root API
or with the new map API. See this comment for a bit more info.