Model descriptor to support relationships

[ngyv]

{
  id: type('number', { required: true }),
  name:  type('string', { default: 'Nurul' }),
  account: belongsTo('account', { id: 'anotherModelKey' }),
  hobbies: hasMany('hobby', { async: true }),
  partner: hasOne('user', { recursive: true })
}

and then handle it in DomainStore:

• [ ] auto fetches relationships if async: false

• [ ] models should have a reference to the other model once it's been fetched.

Note: isDirty will not take into account the state of the other models (relationships).