array_chunks: remove Option wrapper from into_remainder's return type

[hkBst]

This simplifies into_remainder's return type.

[rustbot]

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[scottmcm]

Would it be worth restructuring this to avoid needing the unwrap?

I'm imagining something like

Suggested change

	while self.remainder.is_none() {
	let _ = self.next();
	}
	self.remainder
	self.remainder.unwrap()
	loop {
	if let Some(r) = self.remainder {
	return r;
	}
	let _ = self.next();
	}

[hkBst]

While that is indeed equivalent it has the downside of the loop then being slightly more complicated, but let me know if you want it changed.

@rustbot ready

[WaffleLapkin]

closing this since #149127 was merged.