operator cases

[MiiZZo]

issue #34
New operator that more flexible than variant operator.
Some example of using:

const $user = createStore({ isAdmin: false });

const Component = cases({
  source: $user,
  cases: [
    { view: UserComponent, filter: (user) => !user.isAdmin },
    { view: AdminComponent, filter: (user) => user.isAdmin }, 
  ],
});

const AdminComponent = () => <div>admin></div>
const UserComponent = () => <div>user</user>
const $projects = createStore([]);
const projectCreated = createEvent();

$projects.on(projectCreated, (projects) => [...projects, {}]);

const Component = cases({
  source: $projects,
  cases: [
    { view: CreateYourFirstProject, filter: (projects) => projects.length === 0 },
    { view: ProjectsList, filter: (projects) => projects.length > 0 },
  ],
});

const ProjectsList = () => <list />
const CreateYourFirstProject = () => <button />

I've added an implementation of operator cases and tests for no-ssr but some tests fail and i don't know why.