Upgrade rust/crates/futures-util to 0.3.15

Test: make
Change-Id: Ibac5fc0ece362434ec0af181cfd9860570da7813
diff --git a/src/io/buf_writer.rs b/src/io/buf_writer.rs
index 991a365..f292b87 100644
--- a/src/io/buf_writer.rs
+++ b/src/io/buf_writer.rs
@@ -1,3 +1,4 @@
+use super::DEFAULT_BUF_SIZE;
 use futures_core::ready;
 use futures_core::task::{Context, Poll};
 use futures_io::{AsyncBufRead, AsyncRead, AsyncSeek, AsyncWrite, IoSlice, SeekFrom};
@@ -5,7 +6,6 @@
 use std::fmt;
 use std::io::{self, Write};
 use std::pin::Pin;
-use super::DEFAULT_BUF_SIZE;
 
 pin_project! {
     /// Wraps a writer and buffers its output.
@@ -46,11 +46,7 @@
 
     /// Creates a new `BufWriter` with the specified buffer capacity.
     pub fn with_capacity(cap: usize, inner: W) -> Self {
-        Self {
-            inner,
-            buf: Vec::with_capacity(cap),
-            written: 0,
-        }
+        Self { inner, buf: Vec::with_capacity(cap), written: 0 }
     }
 
     fn flush_buf(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<io::Result<()>> {