Vendor dependencies

Let's see how I like this workflow.

vendor/anyhow/tests/ui/chained-comparison.rs

@@ -0,0 +1,8 @@
+use anyhow::{ensure, Result};
+
+fn main() -> Result<()> {
+    // `ensure!` must not partition this into `(false) == (false == true)`
+    // because Rust doesn't ordinarily allow this form of expression.
+    ensure!(false == false == true);
+    Ok(())
+}