[racket] cool things in Whalesong, part 2

Here's another example that's starting to work, though it isn't flashy yet.

    http://hashcollision.org/tmp/js-get-message/js-get-message-parent.html


I know, I don't have any design sense yet.  I need help.  :)



The first half of the window is a simple web-world program embedded in
an iframe, whose source is:

    http://hashcollision.org/tmp/js-get-message/js-get-message-child.rkt

The second half is just honest-to-goodness plain JavaScript in
js-get-message-parent.html.



Typing a message and submitting at the bottom calls HTML5's
postMessage, which sends a message over to the web-world program.  The
web-world program responds to postMessages, and incorporates it into
its world.


Note that the postMessage support is _not_ built in!  I'm using our
experimental FFI bindings to hook DOM message events into the
web-world.  See the top half of js-get-message-child.rkt for the
minimal postMessage bindings.