Vendor dependencies

Let's see how I like this workflow.

vendor/once_cell/examples/reentrant_init_deadlocks.rs

@@ -0,0 +1,14 @@
+fn main() {
+    let cell = once_cell::sync::OnceCell::<u32>::new();
+    cell.get_or_init(|| {
+        cell.get_or_init(|| 1);
+        2
+    });
+}
+
+/// Dummy test to make it seem hang when compiled as `--test`
+/// See https://github.com/matklad/once_cell/issues/79
+#[test]
+fn dummy_test() {
+    std::thread::sleep(std::time::Duration::from_secs(4));
+}