chore: make futures-util optional #337

Job	Run time
semver	22s
Check Style	13s
features	27s
Test with Miri	46s
Check MSRV (1.63)	3m 33s
Test stable on ubuntu-latest	45s
Test stable on windows-latest	1m 38s
Test stable on macOS-latest	41s
Test beta on ubuntu-latest	35s
Test beta on windows-latest	1m 9s
Test beta on macOS-latest	1m 11s
Test nightly on ubuntu-latest	37s
Test nightly on windows-latest	1m 19s
Test nightly on macOS-latest	49s
Build docs	14s
CI is green	0s
	14m 19s

Provide feedback