Add Read::initializer.

This is an API that allows types to indicate that they can be passed
buffers of uninitialized memory which can improve performance.

src/libstd/sys/unix/net.rs

@@ -198,10 +198,6 @@ impl Socket {
         self.recv_from_with_flags(buf, MSG_PEEK)
     }
 
-    pub fn read_to_end(&self, buf: &mut Vec<u8>) -> io::Result<usize> {
-        self.0.read_to_end(buf)
-    }
-
     pub fn write(&self, buf: &[u8]) -> io::Result<usize> {
         self.0.write(buf)
     }