Enable returning `&JsValue` from exported functions

[alexcrichton]

Right now you can't write a function like:

#[wasm_bindgen]
#[no_mangle]
pub extern fn foo(a: &JsValue) -> &JsValue {
    a
}

This is because JsValue goes through all the same machinery as Foo, but I think we want to add a specific trait for "can be returned as a reference" or maybe even just AsRef<JsValue> as I think that's the only way this can work?

Enabling this would help avoid an unnecessary Clone otherwise to extract data from wasm back into JS.

[alexcrichton]

Right now this is blocked by this panic and I think the trait-based solution is probably the best way to go here, IntoWasmAbi I'm pretty sure isn't sound and seems likely to cause use-after-free