Investigate adding std::any::Any to instance

[jonnyboyC]

I'm thinking there may be some benefit to adding some generic fields to the Instance struct. Something like

#[derive(PartialEq, Clone)]
pub struct Instance {
  pub class: Managed<Class>,
  fields: Box<[Value]>,
  native_data: Option<Managed<ManagedAny>>
} 

I think this gives us some options to store extra data related to native functionality without need several new types to values. The example in mind is the beginnings of the regexp module. Here I'm primarily just wrapping the regex crate. For each of the three test methods I have test, match and captures I have currently construct a new Regex struct each time this is invoked which is quite wasteful. Instead I could make a new trait ManagedAny something like

trait ManagedAny : Manage + any::Any {}

This way I can store essentially arbitrary extra data on an instance such as a Regex struct and implement the normal Manage members. Inside methods on my Regexp class I should be able to call.

let instance = this.unwrap().to_instance();
let regex = match instance.native_data.downcast_ref::<Regex>().expect("Expected Regex")
// use regex