Add interfaces for MVI & extended concepts

[chikamichi]

• Actions => {[key: string]: Stream<any>}
• Model => (actions: Actions, state$?: MemoryStream<State>): Stream<Reducer>
• View => (sources: Sources): Stream<VNode>
• Intent => (domSource: DOMSource): Actions
• NAP => (state$: MemoryStream<State>): Actions

[chikamichi]

Refactor related code, such as for instance a view is defined with:

const View: View = state$ => {
  return state$.map(…);
}

export default View

etc.

[chikamichi]

Implemented in 6219a7d2878834a81c68f585b284ac909f136eb7.