Vendor dependencies

Let's see how I like this workflow.

vendor/thiserror/tests/ui/bad-field-attr.rs

@@ -0,0 +1,7 @@
+use thiserror::Error;
+
+#[derive(Error, Debug)]
+#[error(transparent)]
+pub struct Error(#[error(transparent)] std::io::Error);
+
+fn main() {}