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/index.markdown

@@ -6,3 +6,4 @@ Provides access to operating system streams and the file system.
 * [File](file.html)
 * [Stat](stat.html)
 * [Stdin](stdin.html)
+* [Stdout](stdout.html)

doc/site/modules/io/stdin.markdown

@@ -29,9 +29,17 @@ been received.
 
 Returns the byte value as a number or `null` if stdin is closed.
 
+Note that output is not automatically flushed when calling this. If you want to
+display a prompt before reading input, you'll want to call `Stdout.flush()`
+after printing the prompt.
+
 ### **readLine**()
 
 Reads one line of input from stdin. Blocks the current fiber until a full line
 of input has been received.
 
 Returns the string of input or `null` if stdin is closed.
+
+Note that output is not automatically flushed when calling this. If you want to
+display a prompt before reading input, you'll want to call `Stdout.flush()`
+after printing the prompt.

doc/site/modules/io/stdout.markdown

@@ -0,0 +1,11 @@
+^title Stdout Class
+
+The standard output stream.
+
+## Static Methods
+
+### **flush()**
+
+Flushes all buffered data to the stream. Ensures any data written to stdout
+that is in the buffer gets written to the file or terminal that stdout is
+connected to.

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>