feat!: add support for bounded/unbounded mailboxes

[tqwewe]

Closes #3 and #28

This PR changes how the fundamentals of kameo works, but adds support for choosing the boundedness of actor mailboxes. The actor mailbox can be set when implementing Actor with the type Mailbox = BoundedMailbox<Self> or UnboundedMailbox<Self> associated type.

As unbounded channels differ in asyncness than bounded channels, a new builder syntax has been introduced when sending messages to actors. The types are AskRequest, TellRequest, and QueryRequest. These are returned an ActorRef's with the .ask(...), .tell(...), .query(...) methods, and support customizing timeouts.

Sending a message with a reply would look like my_actor_ref.ask(Foo { ... }).send().await?;, and sending a message without a reply looks like my_actor_ref.tell(Foo { ... }).send().await?;