Support final classes

[JukkaL]

Support final class modifier that makes it impossible to inherit from a class. Also make built-in types Int, Str, etc. final. This removes the special status of the built-in types (currently they are the only classes you cannot inherit from).

Example:

final class Foo
  ...
end

class Bar is Foo    -- Error
end

[JukkaL]

Alternatively, use the keyword sealed instead of final.