Support for protected members in classes

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.
Do not see any tests for that, as a matter of fact do not see any tests for interface scenarios:
* implement an interface with a protected property
* increase the visibility of a protected property
* inherit multiple protected properties with the same name
* inherit the same protected property from two interfaces
* inherit the same protected property from two interfaces but through different classes 
* assign class with protected property to interface that inherits from it or one of its bases
* access a protected property off an interface extending its defining base class
* derived class changing the type of the protected member to a  sub-type, then checking assignablity scenarios with base
* derived class changing the type of the protected member to any, then checking assignablity scenarios with base