Allow classnames for Transformer and Serializers

[cviebrock]

This allows passing a classname (i.e. string) anywhere you can specify a Transformer or Serializer.

e.g., this will now work:

Fractal::create($books, BookTransformer::class, ArraySerializer::class)->toArray();

(If this is accepted, I'll make a PR to spatie/laravel-fractal to support the changes there as well.)

[sebastiandedeyne]

Wanted this for a while, thumbs up from me!

[freekmurze]

Another quality PR, thanks Colin! 👍