Vendor dependencies

Let's see how I like this workflow.

vendor/cxx/tests/ui/vec_opaque.stderr

@@ -0,0 +1,23 @@
+error: Rust Vec containing C++ type is not supported yet
+  --> tests/ui/vec_opaque.rs:15:19
+   |
+15 |         fn f() -> Vec<Job>;
+   |                   ^^^^^^^^
+
+error: needs a cxx::ExternType impl in order to be used as a vector element in Vec<Job>
+  --> tests/ui/vec_opaque.rs:11:9
+   |
+11 |         type Job;
+   |         ^^^^^^^^
+
+error[E0271]: type mismatch resolving `<handle::Job as ExternType>::Kind == Trivial`
+  --> tests/ui/vec_opaque.rs:22:14
+   |
+22 |         type Job = crate::handle::Job;
+   |              ^^^ expected enum `Trivial`, found enum `Opaque`
+   |
+note: required by a bound in `verify_extern_kind`
+  --> src/extern_type.rs
+   |
+   | pub fn verify_extern_kind<T: ExternType<Kind = Kind>, Kind: self::Kind>() {}
+   |                                         ^^^^^^^^^^^ required by this bound in `verify_extern_kind`