Constify fold, reduce and last for iterator

[Randl]

These functions only require the addition of [const] in appropriate places, so it feels like almost a trivial change. Most of the others require either consitifying additional traits, const closures, or both.

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[tgross35]

I think we should hold off here until at least next can be made const. Otherwise these are just bounds that aren't really useful now and may need to be changed down the line.

[Randl]

Not sure I follow? next isn't marked as non-const because it doesn't have a default implementation. The only implementation of const Iterator as for now is Option, which does have const next

rust/library/core/src/option.rs

Lines 2476 to 2483 in 1e9be1b

	#[rustc_const_unstable(feature = "const_iter", issue = "92476")]
	impl<A> const Iterator for Item<A> {
	type Item = A;
	#[inline]
	fn next(&mut self) -> Option<A> {
	self.opt.take()
	}