rustdoc: json output failure

```
$ cat vacant.rs
#[link(name = "vacant")]; #[crate_type = "lib"];

$ rustdoc vacant.rs -w json
task '<unnamed>' failed at 'Rust generated JSON is invalid??',
/path/to/librustdoc/rustdoc.rs:319
```

Any library with no `#[no_std]` causes the failure.
This is because `rustdoc::clean::Crate` contains `externs: HashMap<ast::CrateNum, ExternalCrate>` so output is like `{1: (...), 2: (...)}`.
