zbus-2.0.0-beta.4

• Allow external executors.
• MessageStream now the main API to receive messages on the low-level and ensures that no two instances can steal messages from each other.
• Introducing ProxyBuilder, that allows building proxies with various options, while new methods on proxy types take care of creating the with sane defaults.
• Generalize ObjectServer::{at,remove} path errors.
• All low-level message receiving API now deals with Arc<Message>, instead of Message.
• Switch from Error: From<E> to E: Into<Error>.
• Drop lifetimes from MessageSink.
• Fix lifetime of SignalStream returned by azync::Proxy::receive_signal.
• Don't use block_on in Drop::drop implementations.
• azync::Proxy::new now async.
• Drop now unnecessary receive_specific methods.
• Re-export static_assertions crate. This is mainly needed for use by zbus_macros.
• Message::primary_header now infallible.
• Implement Clone for PrimaryHeader.
• Message::disown_fds now takes a non-mutable reference.
• Disown FDs in Proxy::call instead of Proxy::call_method.
• Tie internal executor thread to Connection's lifetime so we don't end up launching thousands of threads.
• Handle error from thread spawning.
• azync::Connection::assign_serial_num now synchronous.
• Fix auth/handshake for *BSD.
• Dependency changes:
  • New:
    • async-channel
    • async-broadcast
    • async-executor
    • async-task
  • Dropped:
    • event-listener
  • Upgraded:
    • futures-core
    • futures-sink
    • nix
    • rand
• Many other fixes and improvements.