StrInterner exposes unsafe functions as safe

The [StrInterner](http://doc.rust-lang.org/syntax/util/interner/struct.StrInterner.html) from `librustc` has two conflicting methods:

```
/// Returns this string with lifetime tied to the interner.
/// Since strings may never be removed from the interner,
/// this is safe.
fn get_ref<'a>(&'a self, idx: Name) -> &'a str;
```

And

```
fn clear(&self);
```
