Implement read_buf for a few more types

Implement read_buf for TcpStream, Stdin, StdinLock, ChildStdout,
ChildStderr (and internally for AnonPipe, Handle, Socket), so
that it skips buffer initialization.

The other provided methods like read_to_string and read_to_end are
implemented in terms of read_buf and so benefit from the optimization
as well.

This commit also implements read_vectored and is_read_vectored where
applicable.

library/std/src/sys_common/net.rs

@@ -4,7 +4,7 @@ mod tests;
 use crate::cmp;
 use crate::convert::{TryFrom, TryInto};
 use crate::fmt;
-use crate::io::{self, ErrorKind, IoSlice, IoSliceMut};
+use crate::io::{self, BorrowedCursor, ErrorKind, IoSlice, IoSliceMut};
 use crate::mem;
 use crate::net::{Ipv4Addr, Ipv6Addr, Shutdown, SocketAddr};
 use crate::ptr;
@@ -272,6 +272,10 @@ impl TcpStream {
         self.inner.read(buf)
     }
 
+    pub fn read_buf(&self, buf: BorrowedCursor<'_>) -> io::Result<()> {
+        self.inner.read_buf(buf)
+    }
+
     pub fn read_vectored(&self, bufs: &mut [IoSliceMut<'_>]) -> io::Result<usize> {
         self.inner.read_vectored(bufs)
     }