Use workspaces in this repo

Also cache on appveyor to speed up builds.
diff --git a/appveyor.yml b/appveyor.yml
index de43884..d52efad 100644
--- a/appveyor.yml
+++ b/appveyor.yml
@@ -19,3 +19,7 @@
 test_script:
   - cargo test
   - cargo run --manifest-path libc-test/Cargo.toml
+
+cache:
+  - target
+  - C:\Users\appveyor\.cargo\registry