Implement custom fold for `WhileSome` #530

Job	Run time
check format	19s
check	21s
test	1m 20s
check (--no-default-features)	22s
msrv	1m 21s
check (--no-default-features --features use_alloc)	17s
check (--all-targets --all-features)	32s
All checks succeeded	2s
	4m 34s

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