In ready_chunks, reserve capacity based on size_hint

Reserving `ready_chunks` limit every time may be too expensive when
the limit is high but only a few number of messages available.

Now rely on `Stream::size_hint` to reserve the capacity.  If
underlying stream implements `size_hint`, this will work great.
Otherwise `ready_chunk` will be somewhat less efficient.

This is better alternative to #2657

Author: stepancheg

futures-util/src/stream/stream/ready_chunks.rs

@@ -6,6 +6,7 @@ use futures_core::task::{Context, Poll};
 #[cfg(feature = "sink")]
 use futures_sink::Sink;
 use pin_project_lite::pin_project;
+use std::cmp;
 
 pin_project! {
     /// Stream for the [`ready_chunks`](super::StreamExt::ready_chunks) method.
@@ -49,7 +50,11 @@ impl<St: Stream> Stream for ReadyChunks<St> {
                 // the full one.
                 Poll::Ready(Some(item)) => {
                     if items.is_empty() {
-                        items.reserve(*this.cap);
+                        // Note we reserve capacity here, not when `items` is created,
+                        // because stream may know the remaining size,
+                        // but items might not be readily available.
+                        let size_hint = this.stream.as_mut().size_hint().0.wrapping_add(1);
+                        items.reserve(cmp::min(*this.cap, size_hint));
                     }
                     items.push(item);
                     if items.len() >= *this.cap {