[plt-dev] Release Announcement for v4.2.3, second call

I still need this...  (BTW, these items come from commit messages, so
there might be things that I missed.)
----------------------------------------------------------------------

* added support for changing toolbar buttons based on the #lang
  line, use that for #lang scribble/base

* 2htdp/universe stuff

* Several new kinds of promises

* added `path-up' to scheme/require

* syntax/parse extensions (~var, ~literal, ~and, ~not, etc)

* Adding check-member-of and check-range

* change _pointer to mean a reference to non-GCed memory; add
  _gcpointer

* New `scheme/vector' library.

* scheme/unsafe/ops? (wasn't mentioned last time)

* Add `in-port', `port->list', `file->list'.

* upgrade Boehm GC to v7.1

----------------------------------------------------------------------
-- 
          ((lambda (x) (x x)) (lambda (x) (x x)))          Eli Barzilay:
                    http://barzilay.org/                   Maze is Life!