Implement widget updates

[postsolar]

Closes #3

It works, but the API still need some polishing:

  let
    label /\ updateLabel = W.label'
      { label: asOneOf "abcdef"
      }

  void $ setInterval (Milliseconds 2000.0) do
    log "updating"
    updateLabel \w → w
      { label = U.toEither1 w.label # lmap reverse # either asOneOf asOneOf
      }

No way to bypass untagged union here, but we can add a bifunctorial wrapper to make it a bit nicer.

[postsolar]

A newtype over OneOf a (Binding a) for a bifunctor instance is tricky to implement because it requires that a has a runtime type representation, but it does not show up in instance head. Approaching it from other angles leads to similar issues. Would probably be a lot better to provide a few shortcuts and leave it to the user to use optics here as they're a lot more flexible.