Vendor dependencies for 0.3.0 release

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

@@ -0,0 +1,15 @@
+// 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() {}