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_common/net.rs

@@ -243,10 +243,6 @@ impl TcpStream {
         self.inner.read(buf)
     }
 
-    pub fn read_to_end(&self, buf: &mut Vec<u8>) -> io::Result<usize> {
-        self.inner.read_to_end(buf)
-    }
-
     pub fn write(&self, buf: &[u8]) -> io::Result<usize> {
         let len = cmp::min(buf.len(), <wrlen_t>::max_value() as usize) as wrlen_t;
         let ret = cvt(unsafe {