First stage of struct variant field visibility changes

[sfackler]

We need a snapshot before the parser can be adjusted.

[alexcrichton]

r=me with two more tests (or r+ now if they already exist):

• Some compiler error should happen with pub struct variant fields
• Some test which ensures that private fields cannot be accessed across modules or across crates if they're private (just some error to make sure it's denied)

[sfackler]

pub is still accepted, since the new visibility behavior needs to be staged before the parser can be adjusted.