bufio: optimize bufio.Reader.WriteTo() and bufio.Writer.ReadFrom()

by **valyala**:

<pre>Current implementations of bufio.Reader.WriteTo() and bufio.Writer.ReadFrom() can be
optimized further for common cases such as copying big files to / from network
connections.

See the following topic for details -
<a href="https://groups.google.com/d/msg/golang-nuts/oYYggbLntVE/DMsqEyIU-yIJ">https://groups.google.com/d/msg/golang-nuts/oYYggbLntVE/DMsqEyIU-yIJ</a></pre>
