Vendor dependencies

Let's see how I like this workflow.

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

@@ -0,0 +1,20 @@
+use wasm_bindgen::prelude::*;
+use wasm_bindgen_test::*;
+
+// This test is in the headless suite so that we can test the `externref` table
+// implementation of `externref_heap_live_count` (as opposed to the JS `heap`
+// implementation) in Firefox.
+#[wasm_bindgen_test]
+fn test_externref_heap_live_count() {
+    let initial = wasm_bindgen::externref_heap_live_count();
+
+    let after_alloc = {
+        let _vals: Vec<_> = (0..10).map(JsValue::from).collect();
+        wasm_bindgen::externref_heap_live_count()
+    };
+
+    let after_dealloc = wasm_bindgen::externref_heap_live_count();
+
+    assert_eq!(initial, after_dealloc);
+    assert_eq!(initial + 10, after_alloc);
+}