Add `mutate` to Query model

[tonyhb]

mutate will mutate data before being passed into the component:

@load({
  users: User.getList().mutate(users => {
    const byId = {};
    users.forEach(u => byId[u.id] = u);
    return byId;
  }),
})
class Users extends Component {
}

Each mutator can be kept in a separate file for reusability. We should let mutator functions chain to keep composability of mutator functions optimal. Can we also automatically memoize these functions, or store the resulting data in a reducer?