Don't automatically flush on every System.write().

It's handy because it ensures writes are flushed to the terminal before
any calls to read from stdin, but it's also gratuitously slow.

Instead, added a Stdout class with an explicit flush() method that can
be called by the user.

Fix #445.

doc/site/modules/io/template.html

@@ -32,6 +32,7 @@
         <li><a href="file-flags.html">FileFlags</a></li>
         <li><a href="stat.html">Stat</a></li>
         <li><a href="stdin.html">Stdin</a></li>
+        <li><a href="stdout.html">Stdout</a></li>
       </ul>
     </section>
   </nav>
@@ -56,6 +57,7 @@
           <ul>
             <li><a href="stat.html">Stat</a></li>
             <li><a href="stdin.html">Stdin</a></li>
+            <li><a href="stdout.html">Stdout</a></li>
           </ul>
         </td>
       </tr>