Prototype inheritance

[eggyal]

Prototype implementation for draft RFC. Fixes #210 and fixes #1721.

Enables one to do stuff like this—

#[wasm_bindgen(prototype=web_sys::HtmlElement)]
struct MyCustomElement {}

#[wasm_bindgen]
impl MyCustomElement {
    #[wasm_bindgen(constructor)]
    fn new() -> WasmType<MyCustomElement> {
        instantiate! { MyCustomElement{} }
    }
}

// ...

web_sys::window()
    .unwrap()
    .custom_elements()
    .define("my-custom-element", &js_sys::JsFunction::of::<MyCustomElement>())?;

// ...

See Exporting a struct to JS for detailed information, and the examples prefixed web-components- for use-cases.

[alexcrichton]

I'm going to close this while the RFC makes progress to help clean out PRs.