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/main.rs

@@ -10,3 +10,4 @@ pub mod namespace;
 pub mod simple;
 pub mod throws;
 pub mod dictionary;
+pub mod global;