Add variant method

[sergeysova]

Like a forest#variant

const FormDescription = variant({
  source: $pending,
  cases: {
    true: Alert,
    false: Info,
  },
})

const Panel = variant({
  source: $error,
  bind: {
    text: $text,
  },
  cases: {
    username_is_invalid: Alert,
    username_is_abandoned: Info,
    ok: Success,
  },
})

const Alert = reflect({
  bind: {
    type: $store.map(fn),
  },
  view: Notification,
})

const Field = variant({
  source: $field.map(field => field.type),
  bind: {
    text: $text,
  },
  cases: {
    text: Input,
    number: Range,
    boolean: Checkbox,
    select: Select,
    field: Textarea,
  },
  default: () => null,
})

function variant({ source, cases, bind, default: Default }) {
	return (props) => <Default />
}