A simple event bus that can be cloned and shared across threads.

## Emit inside `on` blocks
To re-emit events inside `on` closures do not clone the original bus, use the bus
reference provided to the closure instead.

## Example

```rust
use tram::{prelude::*, unsync::EventBus};
use std::{rc::Rc, cell::RefCell};

#[derive(PartialEq, Eq, Hash)]
enum EventType {
    Start,
    Stop,
}

#[derive(Debug, PartialEq)]
enum Status {
    Stopped,
    Started,
}

let bus: EventBus<EventType, ()> = EventBus::unbound();

let status = Rc::new(RefCell::new(Status::Stopped));
let status_closure = Rc::clone(&status);
let status_closure_2 = Rc::clone(&status);

bus.on(EventType::Start, move |_bus, _| {
    *status_closure.borrow_mut() = Status::Started;
})
.unwrap();

bus.on(EventType::Stop, move |_bus, _| {
    *status_closure_2.borrow_mut() = Status::Stopped;
})
.unwrap();

bus.emit(EventType::Start).expect("Failed to emit");

assert_eq!(*status.borrow(), Status::Started);
assert_eq!(bus.event_count(), 1);

bus.emit(EventType::Stop).expect("Failed to emit");

assert_eq!(*status.borrow(), Status::Stopped);
assert_eq!(bus.event_count(), 2);
```

## Using it in threads

```rust
use tram::{prelude::*, sync::EventBus};
use std::sync::{Arc, Mutex};

#[derive(PartialEq, Eq, Hash)]
enum EventType {
    Start,
    Stop,
}

#[derive(Debug, PartialEq)]
enum Status {
    Stopped,
    Started,
}

let bus: EventBus<EventType, ()> = EventBus::unbound();
let bus_clone = bus.clone();

let status = Arc::new(Mutex::new(Status::Stopped));
let final_status = Arc::clone(&status);

let t1 = std::thread::spawn(move || {
    bus.on(EventType::Start, move |_bus, _| {
        let mut status_lock = status.lock().unwrap();
        *status_lock = Status::Started;
    }).unwrap();
});

let t2 = std::thread::spawn(move || {
    bus_clone.emit(EventType::Start).unwrap();
});

t1.join().unwrap();
t2.join().unwrap();

let final_status_lock = final_status.lock().unwrap();
assert_eq!(*final_status_lock, Status::Started)
```

## Passing data to events

```rust
use tram::{prelude::*, unsync::EventBus};
use std::{rc::Rc, cell::RefCell};

#[derive(PartialEq, Eq, Hash)]
enum EventType {
    Start,
    Stop,
}

#[derive(Debug, PartialEq)]
enum Status {
    Stopped,
    Started,
}

let bus: EventBus<EventType, u8> = EventBus::unbound();
let status: Rc<RefCell<Option<u8>>> = Rc::new(RefCell::new(None));
let status_closure = Rc::clone(&status);

bus.on(EventType::Start, move |_bus, startup_data| {
    *status_closure.borrow_mut() = Some(*startup_data.unwrap());
})
.unwrap();

bus.emit_with_value(EventType::Start, Some(&123)).expect("Failed to emit");

assert_eq!(*status.borrow(), Some(123));
assert_eq!(bus.event_count(), 1);
```