Update to Racket 5.2

Release Highlights:

* DrRacket comes with an experimental, on-line check syntax tool,
  although this new tool is disabled default.  See below for more
  information.

* The new `db' library offers a high-level, functional interface to
  popular relational database systems, including PostgreSQL, MySQL,
  and SQLite, as well as other systems via ODBC.

* A new XREPL collection provides convenient commands for a plain
  racket REPL.  It is particularly convenient for people who prefer
  console-based work and alternative editors.  See also the new
  chapter on command-line tools and other editors at the end of the
  Racket Guide.

* The `plot' collection has been reimplemented in Racket.  It now
  offers PDF output, log axes, histograms, and more.  Some code that
  uses `plot' will still work, and some will need light porting.
  The `plot/compat' module offers expedient backward compatibility.

* DrRacket uses more conventional key bindings: `C-t' creates a new
  tab, `C-w' closes the current one, and `C-r' runs the definitions.
  On Mac OS X, the Command key is used.  See "Defining Custom
  Shortcuts" in the DrRacket manual for an example that uses the old
  key bindings.

* The new `raco link' command registers a directory as a collection,
  which allows the collection directory to reside outside the
  "collects" tree and without changing the PLTCOLLECTS environment
  variable.

* Typed Racket:
  - Typed Racket provides static performance debugging support to
    show which code gets optimized and point out code that does not.
    Use the "Performance Report" button in DrRacket.
  - More intuitive types in printouts in the REPL and in error
    messages.  Use `:query-result-type' to explore types, or
    `:print-type' for a full printout.
  - Typed Racket now supports defining function with optional
    arguments using the same syntax as Racket.

* Redex now supports specifying (and testing and automatically
  typesetting) judgment forms including type systems and SOS-style
  operational semantics.

* Fixed several GUI problems, including problems on Ubuntu 11.10
  (GTK+ 3) and 64-bit Mac OS X.

* Internal-definition expansion has changed to use `let*' semantics
  for sequences that contain no back references.  This change
  removes a performance penalty for using internal definitions
  instead of `let' in common cases, and it only changes the meaning
  of programs that capture continuations in internal definitions.
  Internal definitions are now considered preferable in style to
  `let'.

* Support for `begin-for-syntax' has been generalized; modules may
  now define and export both value bindings and syntax bindings
  (macros) at phase 1 and higher.

  Due to a bug, phase 1 syntax (or higher) is not available in
  DrRacket's `#lang'-based REPL.  A simple workaround is to disable
  debugging in DrRacket (see "no debugging" radio button in detailed
  language dialog).


Additional Items:

* The `racket/gui' library (and Slideshow) provides more support for
  multiple-screen displays.

* DrRacket remembers whether an opened file used LF or CRLF line
  endings, and will continue using the same.  When creating a new
  file, a preference determines how it is saved.

* `net/url' can now follow HTTP redirections.

* The LNCS and JFP class files are no longer distributed with
  Racket.  Instead, they are downloaded on demand.

* The Algol language implementation is now available as a plain
  language using `#lang algol60'.

* The Racket-to-C compiler (as accessed via `raco ctool' or `mzc')
  has been removed; Racket's JIT has long provided better
  performance, and the FFI provides better access to C libraries.

* Contracts can be applied to exports with the new `contract-out'
  form within `provide', instead of a separate `provide/contract'
  form.  (The new `contract-out' form is implemented as a new kind
  of "provide pre-transformer".)

* The `date*' structure type is an extension of `date' with
  `nanosecond' and `time-zone-name' fields.

* New looping constructs: `for/sum' and `for/product'.

* Direct calls to keyword-accepting functions are now optimized to
  eliminate the overhead of keywords.  In addition, the compiler
  detects and logs warnings for keyword-argument mismatches.

* The libfit interface is available from `plot/deprecated/fit', and
  will be removed in the near future.

* The Unix installer has been re-done, and it is now more robust.

* The built-in reader and printer support for Honu is removed.
  (This functionality is re-implemented in Racket.)


On-line Check Syntax:

DrRacket now provides an on-line version of the syntax check tool,
which means that syntax checking runs automatically while you
continue to edit a program.  With this tool enabled, its annotations
(e.g., binding arrows) and actions (e.g., the renaming refactoring
and direct documentation links) are almost always available.

We have noticed that on-line syntax checking renders DrRacket
unstable on occasion, perhaps because it relies on relatively new
support for parallelism.  Occurrences of the problem are rare, but
they are not rare enough, which is why we have disabled the tool by
default.  At the same time, current users of the tool find it so
valuable that we felt it should be included in the release.  We
expect to track down the remaining problems and enable the tool by
default in near-future release.

To enable on-line syntax checking (for `#lang'-based programs only),
click on the red dot in the bottom right of DrRacket's window.  To
turn it off, click there again.

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