Add AppVeyor configuration file.

See: arti#450

.appveyor.yml

@@ -0,0 +1,57 @@
+image:
+  - Visual Studio 2019
+
+environment:
+  matrix:
+    - channel: stable
+      target: x86_64-pc-windows-msvc
+
+    - channel: stable
+      target: i686-pc-windows-msvc
+
+    - channel: beta
+      target: x86_64-pc-windows-msvc
+
+    - channel: beta
+      target: i686-pc-windows-msvc
+
+    - channel: nightly
+      target: x86_64-pc-windows-msvc
+
+    - channel: nightly
+      target: i686-pc-windows-msvc
+
+    - channel: stable
+      target: x86_64-pc-windows-gnu
+
+    - channel: stable
+      target: i686-pc-windows-gnu
+
+    - channel: beta
+      target: x86_64-pc-windows-gnu
+
+    - channel: beta
+      target: i686-pc-windows-gnu
+
+    - channel: nightly
+      target: x86_64-pc-windows-gnu
+
+    - channel: nightly
+      target: i686-pc-windows-gnu
+
+matrix:
+  allow_failures:
+    - channel: nightly
+
+install:
+  - appveyor DownloadFile https://win.rustup.rs/ -FileName rustup-init.exe
+  - rustup-init -yv --default-toolchain %channel% --default-host %target%
+  - set PATH=%PATH%;%USERPROFILE%\.cargo\bin
+  - rustc -vV
+  - cargo -vV
+  - rustup show
+
+build: false
+
+test_script:
+  - cargo test --verbose %cargoflags%