Do not allocate capacity in ready_chunks

`read_chunks` practically often returns items when not all capacity
filled.  So always allocating full capacity results in excessive
allocation.

This is especially an issue when `capacity` parameter passed is
large (to be able to handle streams efficiently under high load).

Author: stepancheg

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

@@ -24,11 +24,7 @@ impl<St: Stream> ReadyChunks<St> {
     pub(super) fn new(stream: St, capacity: usize) -> Self {
         assert!(capacity > 0);
 
-        Self {
-            stream: super::Fuse::new(stream),
-            items: Vec::with_capacity(capacity),
-            cap: capacity,
-        }
+        Self { stream: super::Fuse::new(stream), items: Vec::new(), cap: capacity }
     }
 
     delegate_access_inner!(stream, St, (.));
@@ -48,7 +44,7 @@ impl<St: Stream> Stream for ReadyChunks<St> {
                     return if this.items.is_empty() {
                         Poll::Pending
                     } else {
-                        Poll::Ready(Some(mem::replace(this.items, Vec::with_capacity(*this.cap))))
+                        Poll::Ready(Some(mem::take(this.items)))
                     }
                 }
 
@@ -58,10 +54,7 @@ impl<St: Stream> Stream for ReadyChunks<St> {
                 Poll::Ready(Some(item)) => {
                     this.items.push(item);
                     if this.items.len() >= *this.cap {
-                        return Poll::Ready(Some(mem::replace(
-                            this.items,
-                            Vec::with_capacity(*this.cap),
-                        )));
+                        return Poll::Ready(Some(mem::take(this.items)));
                     }
                 }