Release v0.6.0

Changes

This release contains mostly internal changes that should improve the developer experience of
people working on the VM, but also adds some cool new features.

VM

• Processes now have a more general abstraction, the Process trait. It allows us to treat
  anything that can receive a "message" as a process. At the moment this can only be a WebAssembly
  process or native Rust closures, but it could be extended in the future with other resources
  that act as processes.

• Tags were added to messages, allowing for selective receives. A common use case for them is to
  make a request to a process and ignore all other messages until the response arrives. This can
  be now done by giving the request message a specific tag (i64 value) and waiting for a response
  on that tag with lunatic::message::receive. The receive function will first search the
  existing mailbox for the first message matching the tag or block until a message with the
  specified tag arrives. If we know that such a tag can't yet exist in the mailbox, we can use the
  atomic send and receive operation (send_receive_skip_search) that will not look through
  the mailbox.

• Messages are now just a special kind of signals that a process can receive. Other signals
  are Kill, Link, Unlink, ...

• A test was added for catching signature changes of host functions.

• The messaging API was extended, including functions write_data and read_data that
  allow for streaming zero-copy message de/serialization.

• The Environment was extended with a concept of a registry and 3 host functions:
  register, unregister and lookup. Processes can now be registered inside the
  Environment under a well known name and version number. When looking up processes inside the
  Environment with a query, the lookup will follow semantic versioning rules for the version.
  If we have a process under the name "test" and version "1.2.3", a lookup query with the name
  "test" and version "^1.2" will match it.

• Fixed an issue around async Rust cancellation safety and receives with timeouts.

• Improved handling of command line arguments and environment variables.

Rust library

• The Message trait was removed and we now solely rely on serde's Serialize & Deserialize
  traits to define what can be a message. Originally I was thinking that this is going to be an
  issue once we get support for Rust's native TcpStream and we can't define serde's traits for
  it, but this can be solved with remote derives in the future. This removes a really big
  and complex macro from the library and allows us to use the new write_data and
  read_data host functions for zero-copy de/serialization.

• MessagePack is now used as the default message serialization format.

• A request/reply API was added, that was built on the new selective receive functionality.

• The Environment struct was extended with the new registry functionality.

• New lunatic::main & lunatic::test macros were added to improve developer experiences.

• lunatic::process::this_env was added to get the environment that the process was spawned
  in.