feat: Avoid spawning async-io thread if possible #975

Job	Run time
wine	1m 55s
msrv (ubuntu-latest)	32s
windows_gnu (nightly, x86_64-pc-windows-gnu)	1m 31s
cross (macos-latest, nightly)	1m 47s
cross (ubuntu-latest, nightly)	2m 43s
cross (macos-latest, stable)	2m 8s
msrv (windows-latest)	36s
cross (ubuntu-latest, stable)	1m 53s
security_audit / security_audit	12s
fmt / fmt	14s
test (macos-latest, beta)	57s
test (windows-latest, nightly)	1m 39s
test (windows-latest, beta)	1m 57s
test (ubuntu-latest, beta)	53s
test (windows-latest, stable)	2m 6s
test (ubuntu-latest, stable)	53s
clippy / clippy	45s
test (ubuntu-latest, nightly)	53s
test (macos-latest, stable)	1m 5s
test (macos-latest, nightly)	1m 8s
	25m 47s

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