v0.4.0

This release changes some major APIs and adds shared pointer support:

#32 Introduces new context traits in place of the previous context struct.
#33 Allows serialization parameterization over the type of the serializer, allowing users to choose the complexity of their serializers and create their own.
#34 Renames Unarchive to Deserialize.
#35 Splits Archive into Serialize to allow a clean hierarchy while still enabling parameterization.
#39 Removes the Resolve trait and simplifies the API.
#37 Performed some cleanup on the rkyv_dyn API.
#36 Added shared and weak pointer support.
#41 Added validation support for shared and weak pointers.
#40 Added deserialization contexts to match serialization and verification contexts.
#47 Introduced Serializers and Deserializers.
#42 Added an attribute to rename the resolver type for archived values.
#51 Fixed a performance regression.
#45 Updated and greatly improved the book's documentation.
#59 Introduced the ptr_meta crate and vastly reduced the number of dangerous transmutations.
#56 Added archive implementations for usize and isize.
#55 Made the support for 32/64-bit much more uniform and consistent.

Thanks to everyone who contributed by asking questions, filing issues, and making feature requests.