Add chunks_exact method

[JosephKyser]

Would like to request an _exact version of the chunks method akin to how slices work. I believe this would be useful functionality.

[mzr]

+1 to that!

[jswrenn]

Like array_chunks? Or something else?

[scottmcm]

I'm unsure how best to represent the tail for this. Slices have it easy, since https://doc.rust-lang.org/nightly/std/primitive.slice.html#method.as_chunks can just split it up-front into the chunks part and the less-than-the-chunk-size tail (rust-lang/rust#76354).

@mzr @JosephKyser Would it be fine for your uses if this only worked on ExactSizeIterators? The simplest version of this would be something like

if self.len() < N { None }
else { Some(array::from_fn(|_| self.inner.next().unwrap())) }

but if it needs to build up state as it go and return that for the tail later it gets much messier.