Vendor dependencies

Let's see how I like this workflow.

vendor/smallvec/tests/macro.rs

@@ -0,0 +1,24 @@
+/// This file tests `smallvec!` without actually having the macro in scope.
+/// This forces any recursion to use a `$crate` prefix to reliably find itself.
+
+#[test]
+fn smallvec() {
+    let mut vec: smallvec::SmallVec<[i32; 2]>;
+
+    macro_rules! check {
+        ($init:tt) => {
+            vec = smallvec::smallvec! $init;
+            assert_eq!(*vec, *vec! $init);
+        }
+    }
+
+    check!([0; 0]);
+    check!([1; 1]);
+    check!([2; 2]);
+    check!([3; 3]);
+
+    check!([]);
+    check!([1]);
+    check!([1, 2]);
+    check!([1, 2, 3]);
+}