Introduce differentiated constructors

[xaviervia]

Since stateless and stateful React components require different strategies when composing them, it seems reasonable to have different data constructors for the two types.

The proposal is to have

import { Stateless, Stateful } from 'react-dream'

const Pure = Stateless(props => <h1>Hello</h1>)
const ClassComponent = Stateful(class extends Component { ... })

Each constructor would have different implementations for contramap and any other function that requires discriminating between the two.

Additions

• toStateless: that will be a no-op for Stateless, but will wrap the Stateful with a function and return a Stateless instance
• cata and synonym either: case analysis, based on the Fantasy Land type proposals Add types fantasyland/fantasy-land#281 and Add polymorphic type catamorphisms fantasyland/fantasy-land#280 respectively
• match for case analysis with tags (.match({ Stateless: x => x, Stateful: x => x }))
• isStateless : ReactDream -> Boolean
• The main ReactDream entrypoint will detect the component type and use the corresponding data constructor.

To do

• Fully implement and test the data constructors
• Test contramap for both implementations separately
• Implement the detection of the React component type in the main ReactDream function
• Implement toStateless
• Implement cata + either
• Implement match