Implement WebIDL callback interfaces

This commit implements callback interfaces for WebIDL, the final WebIDL
construct that we were unconditionally ignoring! Callback interfaces are
implemented as dictionaries of callbacks. Single-operation callback interfaces
are also expanded when flattening to accept a `Function` as well, in accordance
with the WebIDL spec.

New features have been added to `web-sys` for all the new callback interface
types. Additionally the `EventTarget.webidl` was tweaked to not have
`EventListener?` as this is required for all functional usage and there's no
need to keep that sort of web browser compat here.

Closes #258

crates/webidl-tests/callbacks.js

@@ -0,0 +1,4 @@
+global.TakeCallbackInterface = class {
+  a() {}
+  b() {}
+};

crates/webidl-tests/callbacks.rs

@@ -0,0 +1,38 @@
+use wasm_bindgen_test::*;
+use js_sys::Function;
+
+include!(concat!(env!("OUT_DIR"), "/callbacks.rs"));
+
+#[wasm_bindgen_test]
+fn multi_op_same_name() {
+    let a = CallbackInterface2::new();
+    let b = TakeCallbackInterface::new().unwrap();
+    b.b(&a);
+}
+
+#[wasm_bindgen_test]
+fn single_op_function() {
+    let a = Function::new_no_args("");
+    let b = TakeCallbackInterface::new().unwrap();
+    b.a_with_callback(&a);
+}
+
+#[wasm_bindgen_test]
+fn single_op_dict() {
+    let a = CallbackInterface1::new();
+    let b = TakeCallbackInterface::new().unwrap();
+    b.a_with_callback_interface1(&a);
+}
+
+#[wasm_bindgen_test]
+fn dict_methods() {
+    let mut a = CallbackInterface1::new();
+    a.foo(&Function::new_no_args(""));
+}
+
+#[wasm_bindgen_test]
+fn dict_methods1() {
+    let mut a = CallbackInterface2::new();
+    a.foo(&Function::new_no_args(""));
+    a.bar(&Function::new_no_args(""));
+}

crates/webidl-tests/callbacks.webidl

@@ -0,0 +1,14 @@
+callback interface CallbackInterface1 {
+  void foo();
+};
+
+callback interface CallbackInterface2 {
+  void foo();
+  void bar();
+};
+
+[Constructor()]
+interface TakeCallbackInterface {
+  void a(CallbackInterface1 arg);
+  void b(CallbackInterface2 arg);
+};

crates/webidl-tests/main.rs

@@ -11,3 +11,4 @@ pub mod simple;
 pub mod throws;
 pub mod dictionary;
 pub mod global;
+pub mod callbacks;