pub/priv qualifiers on individual variants

[catamorphism]

use foo::{f, h};

mod foo {
    pub enum f {
        pub g,
        priv h
    }
}


fn main() {
    let y = h;
}

This shouldn't compile, because h is private. But it's currently accepted. I'm not sure if we want to support pub/priv on variants or not, but if not, we should disallow pub/priv on them altogether.

[nikomatsakis]

You are supposed to be able to mark variants as private so as to control
the conditions in which they are constructed.

[catamorphism]

f459acc fixes the problem (as it turns out, priv wasn't working correctly on top-level items, either).