Add synchronous and asynchronous events

[szymon-zadworny]

This PR adds:

• The Event class bindings
• Synchronization methods:
  • through synchronous .wait() for Queues and Events
  • through asynchronous Futures and .await for Events
  • through barriers (which return an Event) for Queues
• memset bindings
• event dependencies
• examples showcasing synchronous and asynchronous waiting

This PR also finalizes the Buffer safety model - safe Buffers are initialized by default and need to be waited/awaited. You can still create uninitialized Buffers through unsafe uninit methods.

Synchronous waiting:

fn main() {
    let mut queue = Queue::new();
    let mut buffer = queue.alloc_shared::<u32>(10).wait();
}

Asynchronous waiting:

#[tokio::main]
async fn main() {
    let mut queue = Queue::new();
    let mut buffer = queue.alloc_shared::<u32>(10).await;
}

Note: tokio was used as a dev dependency for the async.rs example but the actual Futures are executor-agnostic.

This PR requires the following SYCL extensions for the asynchronous execution implementation:

• sycl_ext_intel_queue_immediate_command_list
• sycl_ext_oneapi_enqueue_barrier