feat: Avoid spawning async-io thread if possible #966

Job	Run time
wine	1m 45s
msrv (windows-latest)	34s
cross (macos-latest, stable)	2m 14s
windows_gnu (nightly, x86_64-pc-windows-gnu)	2m 10s
msrv (ubuntu-latest)	1m 12s
cross (ubuntu-latest, stable)	1m 3s
cross (ubuntu-latest, nightly)	1m 5s
test (windows-latest, beta)	1m 40s
test (ubuntu-latest, stable)	54s
fmt / fmt	12s
test (windows-latest, stable)	1m 41s
clippy / clippy	27s
cross (macos-latest, nightly)	2m 12s
test (macos-latest, nightly)	1m 2s
security_audit / security_audit	21s
test (macos-latest, stable)	1m 20s
test (windows-latest, nightly)	1m 40s
test (ubuntu-latest, beta)	56s
test (ubuntu-latest, nightly)	53s
test (macos-latest, beta)	1m 7s
	24m 28s

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