Implement custom fold for `WhileSome` #548

Job	Run time
test	1m 11s
check	8s
msrv	1m 46s
check (--no-default-features)	21s
check format	8s
check (--no-default-features --features use_alloc)	18s
check (--all-targets --all-features)	33s
All checks succeeded	4s
	4m 29s

Provide feedback