Vendor dependencies

Let's see how I like this workflow.

vendor/thiserror/tests/ui/from-backtrace-backtrace.rs

@@ -0,0 +1,10 @@
+// https://github.com/dtolnay/thiserror/issues/163
+
+use std::backtrace::Backtrace;
+use thiserror::Error;
+
+#[derive(Error, Debug)]
+#[error("...")]
+pub struct Error(#[from] #[backtrace] std::io::Error, Backtrace);
+
+fn main() {}