make async optional #270

Job	Run time
changed-files	4s
changed-files-miri	15s
build-docs	1m 9s
test-on-windows-latest	5m 40s
test-on-windows-latest-aarch64	3m 41s
test-on-macos-latest	1m 36s
test-on-ubuntu-latest	2m 4s
test-on-ubuntu-latest-arm32	1m 18s
test-on-ubuntu-latest-aarch64	1m 24s
test-on-ubuntu-latest-use_meter left_handed	2m 10s
test-on-ubuntu-latest-async	2m 6s
test-on-ubuntu-latest-lightweight async-trait	2m 19s
test-on-ubuntu-latest-dynamic_freq	2m 7s
test-with-miri	0s
all-tests-passed	1s
auto-merge	0s
	25m 54s

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