[new RangeInclusive] Convert to half-open in into_iter if possible

[scottmcm]

The core goal here is to attempt to remove the penalty between

for x in i..=j { … }

and

for x in i..(j+1) { … }

by changing how range::RangeInclusiveIter works.

In legacy::RangeInclusive, there's no opportunity to run a fixup before the loop, and trying in every iteration of the loop wasn't worth it.

But now that we have the range::RangeInclusive vs range::RangeInclusiveIter split, we can!

The approach here is to, in into_iter, attempt to convert from start..=last to start..(last+1). Regardless whether that worked, we always delegate to a normal Range for iteration, just with a cold check for "actually we need to return one more element".

Basically you can think of this as having start..=last do either (start..(last+1)).chain(None) or (start..last).chain(Some(last)), though stored more efficiently than literally using Chain.

That way, for example, for a slice of non-ZST if you run a for i in 0..=slice.len() loop, it runs exactly the same as if you'd written for i in 0..(slice.len() + 1) (while also still working for slize-of-ZST where slice.len() + 1 might overflow).

r? libs