Support for protected members in classes #688
This change adds support for protected members in classes as proposed in #1. Also, this change introduces better error messages for assignability and subtype checks involving private and optional properties.
Protected members are modeled after C# and Java. Protected members can be accessed only within the declaring class and subclasses of the declaring class. Furthermore, access to a protected instance member is required to take place through an instance of the enclosing class type or a class type constructed from it (this prevents "sibling" class access).
Protected members can be made public in derived classes. Similar to private members, constructors and signature members cannot be protected.
I will follow up with a more formal specification of the rules but wanted to put up the code for folks to experiment with.
Update: Formal specification provided in #700.