WIP: Prepare fetch

[andremussche]

Prepared loading per level, so no goroutines per field/object but single threaded processing.
Also no dataloader is needed anymore (dataloaden plugin has strange timing delays and dependencies!), prepared data is loaded parallel (using only goroutines for that part).

Note:

• generator is not updated, only manual edits are done (see todo example: https://github.com/ultraware/gqlgen/tree/prepare-fetch/example/todo)
• no tests added yet
• documentation not updated yet