the `fields` hash is here for these kinds of things.
c.f. the comment above:
```ruby
      # Using a hash because:
      # - at some point, we want to associate informations with a field (like the type)
      # - it gives us a set for free
```

Essentially, you want to do something like:

```ruby
klass.fields[name] ||= {}
klass.fields[name].merge!(options)
```