Specialize `Multipeek::fold` #638

Job	Run time
check format	5s
msrv	26s
test	49s
check	15s
check (--no-default-features)	13s
check (--no-default-features --features use_alloc)	36s
check (--all-targets --all-features)	17s
All checks succeeded	3s
	2m 44s

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