Add static check for capsule cores being passed out of capsule cores

[dwtj]

It is an error for a method of a capsule core:

• pass or return an instance of a capsule core (esp. itself) to another method or procedure.
• pass or return an instance of a capsule to another method or to

Unfortunately, method calls are done at the expression-level, and all expressions are opaque to a fully-portable annotation processor. Therefore, implementing this feature by inspecting the AST of all capsule cores cannot be done in a portable way.

We could check all methods being compiled to make sure that none of them are passing/returning a core type or a capsule type. (I think that it is not too restrictive to assume that pure Java code should not be allowed to pass/return capsule and capsule core types.)

[ddmills]

On a side note - renaming Template to Core is a great improvement! 👍

[dwtj]

Concerning "Template" -> "Core": Thanks, I agree.

We are still trying to think about a better name for what were signature templates and were renamed to signature cores. (We renamed signature templates because it was way easier to do the find-and-replace on both capsules and signatures). Core doesn't make as much sense to me for signatures, since they don't really hold anything. In some sense, they describe the "outside edge" of a capsule.