Vendor dependencies

Let's see how I like this workflow.

vendor/cxx/tests/ui/missing_unsafe.stderr

@@ -0,0 +1,9 @@
+error[E0133]: call to unsafe function is unsafe and requires unsafe function or block
+ --> tests/ui/missing_unsafe.rs:4:12
+  |
+4 |         fn f(x: i32);
+  |            ^        - items do not inherit unsafety from separate enclosing items
+  |            |
+  |            call to unsafe function
+  |
+  = note: consult the function's documentation for information on how to avoid undefined behavior