Add preliminary appveyor support.

appveyor.yml

@@ -0,0 +1,26 @@
+environment:
+  matrix:
+  - TARGET: x86_64-pc-windows-gnu
+    BITS: 64
+    MSYS2: 1
+  - TARGET: x86_64-pc-windows-msvc
+    BITS: 64
+  - TARGET: i686-pc-windows-gnu
+    BITS: 32
+    MSYS2: 1
+  - TARGET: i686-pc-windows-msvc
+    BITS: 32
+install:
+  - curl -sSf -o rustup-init.exe https://win.rustup.rs/
+  - rustup-init.exe -y --default-host %TARGET%
+  - set PATH=%PATH%;C:\Users\appveyor\.cargo\bin
+  - if defined MSYS2 set PATH=C:\msys64\mingw%BITS%\bin;%PATH%
+  - rustc -V
+  - cargo -V
+build: false
+test_script:
+  - cargo build --verbose --all
+  - cargo test --verbose --all
+branches:
+  only:
+    - master