feat(neon): JsFunction::bind() and Object::prop()

[dherman]

This PR implements JsFunction::bind(), which creates a builder for calling a function using the Try{From,Into}Js traits.

let n = f.bind(&mut cx)
    .args((1, 2, 3))?
    .arg(4)?
    .arg_with(|cx| Ok(cx.number(5)))?
    .call::<f64>()?;

It also implements Object::prop(), which creates a builder for accessing object properties using the Try{From,Into}Js traits.

let x: f64 = obj
    .prop(&mut cx, "x")
    .get()?;

obj.prop(&mut cx, "y")
    .set(x)?;

let s: String = obj.prop(&mut cx, "toString")
    .bind()?
    .call()?;

Finally, it offers a shorthand convenience method Object::method(), which extracts an object property and binds it as a method:

let s: String = obj.method(&mut cx, "toString").call()?;

[kjvalencik]

The code and API look great! A couple small suggestions for the doc examples, but it's ready to go!

[kjvalencik]

I don't think this one should be deprecated. It's the low level function that doesn't have the overhead of casting and deserialization. I think we should keep it.