Private struct fields by default

This is an implementation of a portion of [RFC #4](https://github.com/rust-lang/rfcs/blob/master/active/0004-private-fields.md). This PR makes named struct fields private by default (as opposed to inherited by default).

The only real meaty change is the first commit to `rustc`, all other commits are just fallout of that change.

Summary of changes made:

* Named fields are private by default *everywhere*
* The `priv` keyword is now default-deny on named fields (done in a "lint" pass in privacy)

Changes yet to be done (before the RFC is closed)

* Change tuple structs to have private fields by default
* Remove `priv` enum variants
* Make `priv` a reserved keyword