Implement `AsRawFd`/`AsFd` for `EventLoop<'l, Data>` #507

Job	Run time
lint	1m 29s
CI (1.63.0)	4m 29s
CI (stable)	1m 50s
CI (beta)	2m 0s
Documentation on Github Pages	33s
Coverage (ubuntu-latest)	1m 10s
Coverage (macos-latest)	1m 23s
	12m 54s

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