Use proxies to allow for property access hook

[bsouthga]

It could be interesting if we modeled all property access in Documents through Proxies, that way we could add additional metadata / hooks on lookup.

For example, we could add a deprecated flag to a field which would then issue a console.warn('user.name is deprecated, use user.displayName instead!').

new Collection({
  ...
  name: 'user',
  fields: {
    ...
    name: { is: 'string', deprecated: true, replacement: 'displayName' },
    displayName: { is: 'string' }
  }
})

An example of the change to tyranid itself would be something like this...

//...

async findOne(...args) {
 // ... normal find logic

  var handler = {
      get: function(target, name){
          if (name in this.def.deprecatedFields) {
            console.warn(`property ${name} is deprecated, use ${this.def.deprecatedFields[name].replacement} instead.`);
          }
          return target[name];
      }
  };

  return new Proxy(doc, handler);
}

//...