Optimize #[deriving(ToStr)] for C-style enums

This would simplify the pattern like

``` rust
enum Foo {
    A,
    B,
    C
}

fn foo_to_str(f: Foo) -> &'static str {
    match f {
        A => "A",
        B => "B",
        C => "C"
    }
}
```

instead of doing all the fancy reflection.
