Implement `MergeBy::fold` #879

Job	Run time
check format	4s
doc	18s
check	26s
msrv	41s
check (--no-default-features)	21s
semver-checks	2m 38s
check (--no-default-features --features use_alloc)	18s
test	1m 5s
check (--all-targets --all-features)	21s
All checks succeeded	1s
	6m 13s

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