Add private() function

private
-------
This function sets the current class or definition as private.
Calling the class or definition from outside the current module will fail.

*Examples:*

    private()

called in class `foo::bar` will output the following message if class is called
from outside module `foo`:

    Class foo::bar is private

You can specify the error message you want to use as a parameter:

    private("You're not supposed to do that!")

- *Type*: statement