add new benchmark for `FutureGroup` #272

Job	Run time
Build and test (miri, nightly)	1m 21s
msrv	31s
Checking clippy, fmt and docs	30s
Build and test (ubuntu-latest, stable)	1m 7s
Build and test (windows-latest, stable)	2m 31s
semver-checks	2m 37s
Build and test (macOS-latest, stable)	1m 32s
	10m 9s

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