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

examples/canvas/src/lib.rs

@@ -6,13 +6,9 @@ use std::f64;
 use wasm_bindgen::prelude::*;
 use wasm_bindgen::JsCast;
 
-#[wasm_bindgen]
-extern "C" {
-    static document: web_sys::Document;
-}
-
 #[wasm_bindgen]
 pub fn draw() {
+    let document = web_sys::Window::document().unwrap();
     let canvas = document.get_element_by_id("canvas").unwrap();
     let canvas: web_sys::HtmlCanvasElement = canvas
         .dyn_into::<web_sys::HtmlCanvasElement>()