Ergonomic function-calling API

[dherman]

This PR implements an ergonomic function-calling API using an options object and method chaining. It offers two new methods, JsFunction::call_with() and JsFunction::construct_with(), along with an Arguments trait that enables lightweight syntax for heterogeneously-typed arguments expressed as Rust tuples. It also uses type inference to downcast the result of the function call to an expected type. All together, this makes calling JS functions from Rust much more ergonomic.

Before:

let this = cx.undefined();
let args = vec![
    cx.number(17).upcast(),
    cx.string("hello").upcast()
];
let v: Handle<JsArray> = f.call(&mut cx, this, args)?
    .downcast_or_throw()?;

After:

let v: Handle<JsArray> = f
    .call_with(&cx)
    .args((cx.number(17), cx.string("hello"))
    .apply(&mut cx)?;

Previous attempts:

• Arguments builder API #796
• Modified .call()/.construct() API #816
• Refined .call() and .construct() API #817

Also related:

• Simplify call and construct to only take a AsRef<Handle<JsValue>> that can be directly passed to args #825