Vendor dependencies

Let's see how I like this workflow.

vendor/assert_matches/tests/selective_import2.rs

@@ -0,0 +1,9 @@
+#[macro_use(debug_assert_matches)]
+extern crate assert_matches;
+
+#[test]
+fn test_assert_succeed() {
+    let a = 42u32;
+
+    debug_assert_matches!(a, 42);
+}