Add WebIDL support for the ArrayBuffer type

Should help enable a slew of new bindings as well.

crates/webidl-tests/array_buffer.rs

@@ -0,0 +1,11 @@
+use wasm_bindgen_test::*;
+
+include!(concat!(env!("OUT_DIR"), "/array_buffer.rs"));
+
+#[wasm_bindgen_test]
+fn take_and_return_a_bunch_of_slices() {
+    let f = ArrayBufferTest::new().unwrap();
+    let x = f.get_buffer();
+    f.set_buffer(None);
+    f.set_buffer(Some(x));
+}