Add Async::new_nonblocking and avoid needless set_nonblocking calls #637

Job	Run time
windows_gnu (nightly, x86_64-pc-windows-gnu)	1m 59s
test (ubuntu-latest, nightly)	1m 15s
test (ubuntu-latest, beta)	1m 2s
clippy	31s
cross (ubuntu-latest)	2m 47s
test (ubuntu-latest, stable)	1m 4s
cross (macos-latest)	43s
fmt	19s
test (windows-latest, nightly)	2m 2s
msrv (ubuntu-latest, 1.63)	1m 24s
test (windows-latest, beta)	1m 47s
msrv (windows-latest, 1.63)	1m 47s
test (windows-latest, stable)	2m 4s
security_audit	11s
test (macos-latest, nightly)	1m 58s
test (macos-latest, beta)	1m 36s
test (macos-latest, stable)	1m 23s
	23m 52s

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