Specialize `WithPosition::fold` #494

Job	Run time
msrv	1m 17s
test	1m 12s
check format	19s
check	24s
check (--no-default-features)	16s
check (--no-default-features --features use_alloc)	17s
check (--all-targets --all-features)	33s
All checks succeeded	4s
	4m 22s

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