More generic maps (in `GroupingMap`) #822

Job	Run time
check format	10s
msrv	1m 44s
doc	6s
check	6s
semver-checks	2m 26s
check (--no-default-features)	8s
test	1m 3s
check (--no-default-features --features use_alloc)	7s
check (--all-targets --all-features)	17s
All checks succeeded	0s
	6m 7s

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