feat: Avoid spawning async-io thread if possible #962

Job	Run time
windows_gnu (nightly, x86_64-pc-windows-gnu)	5h 39m 2s
cross (macos-latest, stable)	2m 6s
wine	6h 5m 0s
msrv (ubuntu-latest)	28s
clippy / clippy	28s
cross (ubuntu-latest, stable)	1m 48s
cross (ubuntu-latest, nightly)	6h 5m 0s
cross (macos-latest, nightly)	2m 2s
msrv (windows-latest)	37s
security_audit / security_audit	22s
test (windows-latest, beta)	5h 38m 30s
test (ubuntu-latest, nightly)	55s
test (macos-latest, nightly)	1m 16s
test (windows-latest, stable)	5h 38m 3s
test (macos-latest, beta)	1m 0s
test (ubuntu-latest, beta)	6h 0m 5s
fmt / fmt	14s
test (windows-latest, nightly)	5h 38m 9s
test (macos-latest, stable)	6h 5m 1s
test (ubuntu-latest, stable)	56s
	1d 23h 1m 2s

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