Injections: callbacks should have option to be memoized

[joshjung]

Example code:

export default class FormBaseController extends Controller {
  constructor() {
    super();

    this.injections.formModel = () => {
      this.modelWatcher.find(FormBaseModel);
    };
  }
}

Note that in this case every single time an injection runs, the injector will recompute (find) the FormBaseModel - which might be costly. In some cases this might be required, but I think that we should have a rule that if the injection callback returns a value, then that value that is returned is memoized (cached) for next time the injection is evaluated rather than calling the callback all over again.

Target Code:

export default class FormBaseController extends Controller {
  constructor() {
    super();

    // Our callback will only be called once, and then after that it will reuse the returned value.
    this.injections.formModel = () => this.modelWatcher.find(FormBaseModel);
  }
}

However, if the injection property (formModel in this case) is ever overwritten with a new function or a new value, then the memoized value needs to be discarded so we will have to watch for changes to the injection value (in this case formModel)