refactor(lib): drop futures-util except in ffi (#3890) #3018

Job	Run time
Check Style	15s
semver	21s
Verify hyper.h is up to date	2m 21s
Check MSRV	2m 42s
features	44s
Test stable on windows-latest	2m 19s
Test stable on ubuntu-latest	1m 15s
Test with Miri	2m 46s
Test stable on macOS-latest	1m 13s
Test nightly on ubuntu-latest	1m 10s
Test beta on ubuntu-latest	1m 16s
minimal-versions	35s
Test beta on macOS-latest	1m 15s
Test nightly on macOS-latest	1m 37s
Test beta on windows-latest	2m 31s
udeps	24s
Test cargo-c support (FFI)	24s
Test nightly on windows-latest	2m 36s
Test C API (FFI)	44s
Build docs	23s
Check exposed types	1m 1s
CI is green	3s
	27m 55s

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