consider removing Linkable and LinkableList

[warriordog]

We could substitute them like this:

• Linkable<T> -> ASType
• LinkableList<T> -> List<ASType>

Pros:

• Reduced code complexity
• Eliminate several JSON converters
• Can model something like MentionLink | ASObject

Cons:

• Can no longer model something like ASLink | PersonActor - user would need to do something like if (asType is ASLink link) {} or if (asType is PersonActor person) {}
• We will break ActorEndpoints - it does not derive from ASObject so ASType can't be used as a base

[warriordog]

This will impact #26

[warriordog]

Closing for now - it would be too much work considering that we'd have to re-implement ActorEndpoints