Size hint for Chunks is wrong

[Darksonn]

The size hint for chunks is computed as follows:

fn size_hint(&self) -> (usize, Option<usize>) {
    let chunk_len = if self.items.is_empty() { 0 } else { 1 };
    let (lower, upper) = self.stream.size_hint();
    let lower = lower.saturating_add(chunk_len);
    let upper = match upper {
        Some(x) => x.checked_add(chunk_len),
        None => None,
    };
    (lower, upper)
}

However, this is incorrect. The stream combines many items into one, so it should divide the size hints by the capacity.

[SaltyKitkat]

In #2611, lower in size_hint is fixed. But upper is untouched. I wonder if we should also change x.checked_add(chunk_len) to (x / self.cap).checked_add(chunk_len) or something like that.

[SaltyKitkat]

Maybe size_hint for chunks should be something like this:

    fn size_hint(&self) -> (usize, Option<usize>) {
        let (lower, upper) = self.stream.size_hint();
        let len = self.items.len();
        let lower = lower.saturating_add(len) / self.cap;
        let upper = upper.and_then(|u| u.checked_add(len)).map(|u| u / self.cap);
        (lower, upper)
    }

Edit: and we should use div_ceil instead of /