Timeout completion event implementation

[mxxo]

I am having some trouble with timeout completion events. We've been checking against the event user data, but I don't think that captures all possible timeouts. Timeouts that expire will return -ETIME, which resultify! maps to an error and exit before we can check is_timeout (source).

For the timeout test in #17 (after setting user_data appropriately), I get

running 1 test
test timeout_test ... FAILED

failures:

---- timeout_test stdout ----
Error: Os { code: 62, kind: Other, message: "Timer expired" }
thread 'timeout_test' panicked at 'assertion failed: `(left == right)`
  left: `1`,
 right: `0`: the test returned a termination value with a non-zero status code (1) which indicates a failure', src/libtest/lib.rs:196:5

Should we perhaps construct a new CompletionQueueEvent using the ETIME information? This seems somewhat linked to #22, as when I tried to change resultify! to handle this case, I started getting segmentation faults, possibly from assume_init on Ok events?

[mxxo]

Closing, I believe this is covered by #53.