Specialize `PadUsing::[r]fold` #644

Job	Run time
test	53s
check	7s
check format	7s
msrv	28s
check (--no-default-features)	5s
check (--no-default-features --features use_alloc)	14s
check (--all-targets --all-features)	17s
All checks succeeded	1s
	2m 12s

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