Use workspaces in this repo

Also cache on appveyor to speed up builds.
diff --git a/Cargo.toml b/Cargo.toml
index a863aa6..5dc0ec4 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -16,3 +16,6 @@
 [features]
 default = ["use_std"]
 use_std = []
+
+[workspace]
+members = ["libc-test", "libc-test/generate-files"]