make async optional #266

Job	Run time
changed-files-miri	18s
changed-files	5s
build-docs	2m 31s
test-on-windows-latest	5m 5s
test-on-windows-latest-aarch64	3m 16s
test-on-macos-latest	1m 44s
test-on-ubuntu-latest	2m 7s
test-on-ubuntu-latest-arm32	1m 38s
test-on-ubuntu-latest-aarch64	1m 19s
test-on-ubuntu-latest-use_meter left_handed	1m 59s
test-on-ubuntu-latest-async	2m 6s
test-on-ubuntu-latest-lightweight async-trait	1m 24s
test-on-ubuntu-latest-dynamic_freq	2m 4s
test-with-miri	0s
all-tests-passed	1s
auto-merge	0s
	25m 37s

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