[racket-dev] Release Announcement for v5.2.1

Below is a rough list of additions and changes for v5.2.1. If you
are responsible for a change, please either elaborate it into an
announcement item or tell me if it shouldn't be included in the
announcement. Let me know if I've missed something.

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- racket
   - io now uses poll() or kqueue() when available
   - intern literal strings, bytes, regexps, chars, numbers
   - floating-point ops now use SSE (when available?)
   - cross-module function inlining, racket/performance-hint
   - OS X build defaults to the platform it is built on (i386 vs x86_64)
   - Add a tutorial to the FFI overview
   - racket/class: send/keyword-apply, dynamic-send
   - generalized gcd, lcm to rationals
   - syntax/datum, datum-case, etc
   - improve choice-evt complexity
   - deprecation note for ActiveX part of MysterX

- racket/draw
   - generalized brush transform to apply to stipple
   - racket/draw: generalize "face" in font% to be description
   - added text-outline to dc-path

- drracket
   - No stacktrace for syntax errors in DrRacket
   - change current-directory default init value
   - render convertibles as png (or changed since?)
   - autocompletion tweak
   - check syntax UI improvements?

- typed/racket
   - Add `typecheck-fail' form for explicit type errors.
   - case-> bug fix

- scribble
   - removed distinctness of "\n" generated by at-reader

- plai
   - new gc code

- redex
   - define-judgment, define-relation improvements
   - optimizations, caching
   - cycle check for define-language

- net
   - changes to net/sendmail, net/unihead
   - changes to net/mime
   - synchronizable events (net/websocket/conn)

- plot improvements
   - click to drag and zoom
   - right-click opens editor pop-up menu
   - removed libfit
   - others?

- icons and images package
   - new icons
   - libraries

- teaching languages
   - planet require syntax

- 2htdp/image
   - added bitmap/file

- 2htdp/universe
   - prefab structs
   - added game pad, pad-handler (?)

- 2htdp/planetcute

- db
   - added support for postgresql arrays
   - implement mysql old password support, connect without db
   - fixed sqlite3 crashing bug
   - nested transactions

- in-slice sequences

- DMdA
   - fix infinite recursive signatures
   - record signatures without fields
   - document which signatures work with check-property

- macro-stepper
   - display binding info for more phases
   - don't evaluate program, no repl

- xrepl fixes

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