This removes the `priv` keyword from the language and removes private enum
variants as a result. The remaining use cases of private enum variants were all
updated to be a struct with one private field that is a private enum.
RFC: 0006-remove-priv
Closes#13535
All field or method references within a class must begin with "self." now.
A bare reference to a field or method in the same class will no longer
typecheck.
Allow writing self.f() within a class that has a method f. In a future
commit, this syntax will be required. For now, you can write either
self.f() or f().
I added a "privacy" field to all methods (whether class methods or not),
which allowed me to refactor the AST somewhat (getting rid of the
class_item type; now there's just class_member).
This was resulting in a different error message depending on whether
the private method you were trying to call was in the same crate
or a different one.
