Export MessageHandler to the prelude (#317)

Previous pull request[1] introduced the MessageHandler datatype, but did
not export it in the prelude.

Examples have been updated so that they don't import MessageHandler manually.

[1]: #309

src/bastion/examples/fibonacci_message_handler.rs

@@ -1,4 +1,4 @@
-use bastion::{message::MessageHandler, prelude::*};
+use bastion::prelude::*;
 
 use tracing::{error, info};
 

src/bastion/examples/message_handler_multiple_types.rs

@@ -1,4 +1,3 @@
-use bastion::message::MessageHandler;
 use bastion::prelude::*;
 use std::fmt::Debug;
 use tracing::error;

src/bastion/src/lib.rs

@@ -112,7 +112,7 @@ pub mod prelude {
     pub use crate::errors::*;
     #[cfg(not(target_os = "windows"))]
     pub use crate::io::*;
-    pub use crate::message::{Answer, AnswerSender, Message, Msg};
+    pub use crate::message::{Answer, AnswerSender, Message, MessageHandler, Msg};
     pub use crate::msg;
     pub use crate::path::{BastionPath, BastionPathElement};
     #[cfg(feature = "scaling")]