[racket-dev] Final(?) draft for v5.1.1

I couldn't resist an editing pass to give the bullets a more consistent
style.

Matthias: I re-wrote your universe bullet in an attempt to clarify that
the universe API didn't change --- only the network protocol, if I
understand correctly. Please check.

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Racket version 5.1.1 is now available from

  http://racket-lang.org/

* The new `racket/stream' library provides `stream-first',
  `stream-rest', a lazy `stream-cons', and so on.  Streams are a
  subtype of sequences, so they work in `for' forms.  Some sequence
  generators, such as `in-range', now produce streams.  A
  `racket/sequence' library replaces the old `racket/stream'
  library.

* The new `racket/syntax' library contains facilities useful for
  writing macros.  The new `syntax/srcloc' and `syntax/location'
  libraries provide support for manipulating source locations.

* The `racket/gui' library now supports multi-column list boxes and
  scrolling panels.

* The new `ffi/file' library is useful for writing foreign library
  bindings that cooperate with Racket's security guard mechanism.

* Generators from the `racket/generator' library can now have formal
  arguments that are used when the generator is fired up.

* Single-precision floating-point support is now enabled by default.
  Single-precision floats print differently from their default
  double-precision counterparts, new primitives convert between the two
  precisions, and new reader syntax supports single-precision literals.

* JIT improvements include a small change to aid x86 branch prediction
  on function-call returns, which can speed up some programs
  significantly.

* Typed Racket:
  - The numeric tower has been entirely overhauled.  TR programs can
    now use more precise types than before, and check more numeric
    properties, such as sign or range properties.
  - Fixnum optimizations have been improved and should apply more
    broadly.
  - The performance of the typechecker has been improved.  In
    particular, dispatch on large union types should typecheck much
    faster than before.

* The Stepper can now step through Lazy Racket programs.

* The `racket/future' library includes `fsemaphore' values, the
  `future' primitive no longer freezes futures (so a future can spawn
  new futures), and `future' log messages are more informative.

* PLaneT development links are now version-specific.

* The `2htdp/image' library now includes `overlay/align',
  `underlay/align', `overlay/align/offset' and 'underlay/align/offset'.

* The network protocol for universes in `2htdp/universe' has changed,
  so that v5.1 and its predecessors are incompatible.

* The "DrScheme" application (which simply ran DrRacket in the last
  few releases) has been removed.  The "MrEd" GUI executables for
  Windows and Mac OS X have also been removed, although the "mred"
  console executable remains for Unix and Mac OS X to support old
  scripts.