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)