Ergonomic struct API for Rust

[Wodann]

Currently, it looks like this when marshalling Mun structs to Rust:

let lib_path = env::args().nth(1).expect("Expected path to a Mun library.");

let mut runtime = 
  RuntimeBuilder::new(lib_path)
    .spawn()
    .expect("Failed to spawn Runtime");

  let runtime_ref = runtime.borrow();
  let mut xy: StructRef = invoke_fn!(runtime_ref, "vector2_new", -1.0f32, 1.0f32).unwrap();
  let x: f32 = xy.get("x").unwrap();
  xy.set("x", x * x).unwrap();
  let y = xy.replace("y", -1.0f32).unwrap();
}

We want to make these actions more ergonomic, safe and (potentially) faster by creating strongly typed Rust structs, e.g.:

#[derive(Mun)]
struct Vector2 {
  x: f32,
  y: f32,
}

This would allow us to simplify the previous code sample, e.g. as follows:

let lib_path = env::args().nth(1).expect("Expected path to a Mun library.");

let mut runtime = 
  RuntimeBuilder::new(lib_path)
    .spawn()
    .expect("Failed to spawn Runtime");

  let runtime_ref = runtime.borrow();
  let mut xy: StructRef = invoke_fn!(runtime_ref, "vector2_new", -1.0f32, 1.0f32).unwrap();
  let x: f32 = xy.x();
  xy.set_x(x * x);
  let y = xy.replace_y(-1.0f32);
}

Apart from making programming with Mun simpler, this would allow us to auto-generate functions for verifying the validity of a struct's fields upon Runtime start-up instead of always needing to check validity when you marshal a field.