[plt-scheme] Typed tetris

I've ported my Tetris game (planet dvanhorn/tetris) to the Typed Scheme 
language.  I thought it might be compelling for those interested in 
using Typed Scheme and the Student language with the World teachpack.

To compare the untyped and typed variants see:

http://planet.plt-scheme.org/package-source/dvanhorn/tetris.plt/2/3/tetris.ss
http://planet.plt-scheme.org/package-source/dvanhorn/tetris.plt/2/3/typed-tetris.ss

The transition from HtDP-style contracts (in comments) to Typed Scheme 
types was effortless.  However I had to give up all of my test cases 
since I couldn't make check-expect work in Typed Scheme.  [There is a 
workaround: have an untyped module for test cases that requires the TS 
code and htdp/testing.]

I had to require the needed bindings from the Student Language and the 
World teachpack with appropriate types.  It would be nicer if there were 
typed variants of the teaching languages and teachpacks.

I also had to rewrite the one occurrence of local into a letrec: since 
there is no Typed Scheme local.

David