[WIP] Twig entity embed

Here's the corresponding d.o issue. https://drupal.org/node/2279257

It is built on 8.x-1.x right now instead of the embed-display branch. This PR will probably stay Work-In-Progress until it uses  EntityEmbedDisplay.
Yeah, you're right. I started this code thinking about blocks first. There isn't much benefit to prioritizing it here. I'll switch it.