Vendor dependencies

Let's see how I like this workflow.

vendor/wasm-bindgen/tests/headless/modules.rs

@@ -0,0 +1,12 @@
+use wasm_bindgen::prelude::*;
+use wasm_bindgen_test::*;
+
+#[wasm_bindgen(raw_module = "./tests/headless/modules.js")]
+extern "C" {
+    fn get_five() -> u32;
+}
+
+#[wasm_bindgen_test]
+fn test_get_five() {
+    assert_eq!(get_five(), 5);
+}