Add __dir__() to built-in objects

The REPL added in #80 has built-in autocompletion:

However various built-in objects do not take full advantage of this:

• Actor does not have autocompletion for attributes delegated to the ZRect
• Loaders do not have autocompletion for the objects that can be loaded
• more?

We should implement __dir__() on these classes and others to ensure that autocompletion lists all valid attributes.