🇺🇸United States @rbayliss

Here's the last version I'm gonna roll today. I think this is truly ready for review. A couple open questions:

1. I created a new method called invokeHookMultiple, which should probably be merged into invokeHook. Maybe invokeHook should always be passed an array of entities, then it can determine which hooks get multiple vs single handling?
2. This doesn't change behavior for field_attach_delete(). Should we change this function and hook_field_attach_delete() over to multiple entity handling?

This one should pass. One thing I noticed is that core uses array_keys($entities) to determine id's in the storage controller, so I think we're safe using that determination in the hook_ENTITYTYPE_delete() as well. That change will come in the next patch.

Here's a first pass.