This information is embedded within the algorithm for constructing interfaces
and their prototypes in the section for ECMAScript glue in the WebIDL spec...
This really *should* make the `wasm_bindgen_backend::ast::ImportType::extends`
member from a `Vec<Ident>` into a `Vec<syn::Path>` so that we could use
`js_sys::Object` in the extends field, but that is a huge pain because then the
`ImportedTypes` trait needs to be changed, and all of its implementers, etc...
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
