Improve low level field management

[gaku-sei]

So far the hook exposes some "low level" functions to handle field and their status:

let {addError} = Form.use()

React.useEffect(() => {
  addError("email", "An error")

  None
}, dependencies)

That's really convenient, especially for async validations and reactive updates, but it's also unsafe for 2 reasons:

1. We use a string
2. The errors are added/removed using the default == operator to compare the errors

I think these 2 issues can be addressed separately, I propose to add a cmp function to the Error module passed to the Formidable.Make functor. As for the first issue I don't have any idea, but we might have to develop a small layer on top of this logic to handle async validations.