add new benchmark for `FutureGroup` #274

Job	Run time
Build and test (ubuntu-latest, stable)	59s
Build and test (windows-latest, stable)	3m 7s
Build and test (miri, nightly)	1m 36s
Build and test (macOS-latest, stable)	1m 28s
msrv	32s
Checking clippy, fmt and docs	32s
semver-checks	19s
	8m 33s

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