Raw methods on SQE should be unsafe

[withoutboats]

Uploaded to crates.io 10 minutes ago and already realized a fricken unsoundness in the API.

Using SubmissionQueueEvent::raw_mut I can implement any of the prep methods in safe code:

sqe.raw_mut().addr = my_buffer.as_ptr() as u64;

raw_mut needs to be unsafe on SQE.