Restore `Actor` type.

The Actor type was removed in #277.

The Actor type currently serves as a base class for the five primary Actor types. It does not, however, define any actor specific properties and it would be highly unlikely for an implementation to use the Actor type directly.

The same reasoning would also apply to Document although that type was not removed.

The Actor would be very useful for identifying custom actor types as an actor (using an as:type array). Currently, there is no way to do that unless the actor type is closely related to one of the five primary Actor types. I'm guessing the original thinking was based on single-value types instead of type arrays.

Example:

{
  "@context": [
       "https://www.w3.org/ns/activitystreams",
       "https://server.example/actors"
  ],
  "type":  ["example:ExampleActor", "Actor"],
  "name": "Example actor"
}

Having the Actor type would allow a server to process a document like this more effectively.

Start by reviewing this issue and the removal discussed in #277, then inspect the current Activity Streams specification's type definitions. Done means the specification restores Actor as a usable base type for custom actors in an as:type array, with the example and processing rationale addressed.