More generic maps in `GroupingMap` (v2) #846

Job	Run time
check format	8s
check	14s
doc	17s
check (--no-default-features)	7s
msrv	27s
check (--no-default-features --features use_alloc)	9s
semver-checks	2m 52s
check (--all-targets --all-features)	29s
test	48s
All checks succeeded	1s
	5m 32s

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