Implement `MergeBy::fold` #880

Job	Run time
doc	9s
check	9s
check format	11s
check (--no-default-features)	27s
check (--no-default-features --features use_alloc)	19s
msrv	40s
semver-checks	20s
check (--all-targets --all-features)	38s
test	1m 34s
All checks succeeded	1s
	4m 28s

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