web-sys: Add support for Global-scope methods

This commit adds further support for the `Global` attribute to not only emit
structural accessors but also emit functions that don't take `&self`. All
methods on a `[Global]` interface will not require `&self` and will call
functions and/or access properties on the global scope.

This should enable things like:

    Window::location() // returns `Location`
    Window::fetch(...) // invokes the `fetch` function

Closes #659

crates/webidl-tests/simple.rs

@@ -64,8 +64,7 @@ fn nullable_method() {
 
 #[wasm_bindgen_test]
 fn global_method() {
-    let f = GlobalMethod::new().unwrap();
-    assert_eq!(f.m(), 123);
+    assert_eq!(GlobalMethod::m(), 123);
 }
 
 #[wasm_bindgen_test]