[racket] Whalesong 1.12 released

Hi everyone,


Whalesong 1.12 includes a few more error traps to catch misuse of
web-world primitives; the major change is that using the web-world
handler functions outside the lexical context of a big-bang is now
illegal.  So the following buggy code:

   (define a-view (initial-view ...))
   (big-bang initial-world
                 (initial-view a-view)
                 ...)

is now caught at compile time.  The corrected code would be:

  (define a-view ...)
  (big-bang initial-world
                (initial-view a-view)
                ...)

without the cut-and-paste error that introduced the duplicate use of
initial-view.


---

Changes:

   * Improved stack traces for the web-world view functions.

   * Improve error trapping with web-world handlers.  web-world
handlers can only be used in syntactic context of an enclosing
big-bang.