New appveyor.yml using stack.

Copied, mostly, from stack's repo.

appveyor.yml

@@ -1,21 +1,23 @@
-install:
-- cmd: 'git submodule update --init'
-- ps: |
-    choco install haskellplatform -version 2014.2.0.0 -y
-    # Haskell Platfrom package doesn't update PATH for the current shell instance
+cache:
+- "c:\\sr" # stack root, short paths == fewer problems
+- "%LOCALAPPDATA%\\Programs\\stack"
 
-    $env:Path += ";${env:ProgramFiles}\Haskell Platform\2014.2.0.0\bin"
-    $env:Path += ";${env:ProgramFiles}\Haskell Platform\2014.2.0.0\lib\extralibs\bin"
-    $env:Path += ";${env:ProgramFiles}\Haskell Platform\2014.2.0.0\mingw\bin"
-    cabal update
-    cabal install --only-dependencies --enable-tests -w "C:\Program Files\Haskell Platform\2014.2.0.0\bin\ghc-7.8.3"
+build: off
 
-build_script:
-- ps: |
-    echo "PATH is $env:Path"
-    cabal configure --enable-tests -v2 -w "C:\Program Files\Haskell Platform\2014.2.0.0\bin\ghc-7.8.3"
-    cabal build
+before_test:
+- ps: Invoke-WebRequest "https://github.com/commercialhaskell/stack/releases/download/v0.1.5.0/stack-0.1.5.0-x86_64-windows.zip" -OutFile stack.zip
+- ps: Invoke-WebRequest "https://github.com/fpco/minghc/blob/master/bin/7z.exe?raw=true" -OutFile 7z.exe
+- ps: Invoke-WebRequest "https://github.com/fpco/minghc/blob/master/bin/7z.dll?raw=true" -OutFile 7z.dll
+- 7z x stack.zip
+- move stack.exe.exe stack.exe
+
+clone_folder: "c:\\stack"
+environment:
+  global:
+    STACK_ROOT: "c:\\sr"
 
 test_script:
-- ps: |
-    cabal test
+# The ugly echo "" hack is to avoid complaints about 0 being an invalid file
+# descriptor
+- echo "" | stack --arch i386 --no-terminal --install-ghc test
+