Enable exported Rust structs to specify prototypal inheritance

[eggyal]

Motivation

Because the JS classes generated for exported Rust structs do not currently specify a parent class, they are implicit subclasses of Object—that is, their prototype objects are Object.prototype. However, there are many situations where the parent class (prototype object) of one's exported structs should be something other than this default.

Proposed Solution

I propose a new super = Type attribute for the wasm_bindgen macro, as follows:

#[wasm_bindgen]
pub struct Parent { ... }
#[wasm_bindgen(super=Parent)]
pub struct Child { ... }

// the superclass might be imported from JS
#[wasm_bindgen]
extern "C" {
    #[wasm_bindgen(extends=Grandparent)]
    pub type ImportedParent;
}
#[wasm_bindgen(super=ImportedParent)]
pub struct ChildOfImportedParent { ... }

// or might be a built-in published by the `js_sys` crate
#[wasm_bindgen(super=js_sys::Date)]
pub struct CustomDate { ... }

// or might be a Web IDL published by the `web_sys` crate
#[wasm_bindgen(super=web_sys::XmlHttpRequest)]
pub struct MyAjaxRequest { ... }

In each case, the generated JavaScript class would include the relevant extends clause, and Rust implementations for the DeRef, AsRef, AsMut and From traits would all be automatically generated.

[alexcrichton]

This is similar to #210 I think as well.

AFAIK no one really knows how best to progress on this at this point in terms of what the design for this would look like, but proposals for such a design (fleshed out as an RFC most likely) would be most welcome!