Test fixtures

In `src/__tests__/` folder add `fixtures.ts` file with a dozen different realistic schemas for various scenarios. Use the type builder, e.g:

```ts
export const User = t.object({
  id: t.str,
  name: t.object({
    first: t.str,
  }).optional({
    last: t.str,
  }),
  // etc..
});
```

Cover all node types.

Once done, make sure linter and formatter pass.