Switch to purely namespaced enums

This breaks code that referred to variant names in the same namespace as
their enum. Reexport the variants in the old location or alter code to
refer to the new locations:

```
pub enum Foo {
    A,
    B
}

fn main() {
    let a = A;
}
```
=>
```
pub use self::Foo::{A, B};

pub enum Foo {
    A,
    B
}

fn main() {
    let a = A;
}
```
or
```
pub enum Foo {
    A,
    B
}

fn main() {
    let a = Foo::A;
}
```

[breaking-change]

src/librustdoc/html/render.rs

@@ -32,6 +32,7 @@
 //! for creating the corresponding search index and source file renderings.
 //! These tasks are not parallelized (they haven't been a bottleneck yet), and
 //! both occur before the crate is rendered.
+pub use self::ExternalLocation::*;
 
 use std::collections::{HashMap, HashSet};
 use std::collections::hash_map::{Occupied, Vacant};