js-sys: allow any JsValue to be fallibly converted into a js_sys::Iterator

[fitzgen]

It should do something like

let into_iter = Reflect::get(this, Symbol::iterator())?;
into_iter.call0(this)

where this is the js_sys::Object.

TODO:

• Implement what's described here
• Add a reference section about how to iterate over JS values to the wasm-bindgen guide

[fitzgen]

To be clear, I'm imagining somethign like a blanket TryFrom impl for js_sys::Iter, but since TryFrom isn't stable, it can be its own generic method or something.