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/doctree.rs

@@ -10,6 +10,8 @@
 
 //! This module is used to store stuff from Rust's AST in a more convenient
 //! manner (and with prettier names) before cleaning.
+pub use self::StructType::*;
+pub use self::TypeBound::*;
 
 use syntax;
 use syntax::codemap::Span;