Specialize `MapForGrouping::fold` #771

Job	Run time
check format	14s
msrv	57s
check	15s
doc	14s
check (--no-default-features)	14s
semver-checks	2m 25s
check (--no-default-features --features use_alloc)	14s
test	56s
check (--all-targets --all-features)	26s
All checks succeeded	1s
	5m 56s

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