Class inheritance, public visibility, etc

New features:

• Types, global variables and class fields are now visible only to the file they were declared.
• Added the "pub" keyword to make them visible everywere.
• Added the class constructor syntax.
• Added class and foreign inheritance.
• Added "null" value for classes and foreigns.
• All variables are now automatically initialized with a default value if they have no initializer.
• Auto variables ("let") must be initialized at declaration.
• And some fixes.