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/unsupported/net.rs

@@ -1,5 +1,5 @@
 use crate::fmt;
-use crate::io::{self, IoSlice, IoSliceMut};
+use crate::io::{self, BorrowedCursor, IoSlice, IoSliceMut};
 use crate::net::{Ipv4Addr, Ipv6Addr, Shutdown, SocketAddr};
 use crate::sys::unsupported;
 use crate::time::Duration;
@@ -39,6 +39,10 @@ impl TcpStream {
         self.0
     }
 
+    pub fn read_buf(&self, _buf: BorrowedCursor<'_>) -> io::Result<()> {
+        self.0
+    }
+
     pub fn read_vectored(&self, _: &mut [IoSliceMut<'_>]) -> io::Result<usize> {
         self.0
     }

library/std/src/sys/unsupported/pipe.rs

@@ -1,4 +1,4 @@
-use crate::io::{self, IoSlice, IoSliceMut};
+use crate::io::{self, BorrowedCursor, IoSlice, IoSliceMut};
 
 pub struct AnonPipe(!);
 
@@ -7,6 +7,10 @@ impl AnonPipe {
         self.0
     }
 
+    pub fn read_buf(&self, _buf: BorrowedCursor<'_>) -> io::Result<()> {
+        self.0
+    }
+
     pub fn read_vectored(&self, _bufs: &mut [IoSliceMut<'_>]) -> io::Result<usize> {
         self.0
     }