(Non) implicitly copyable strings

Str is not implicitly copyable so using them often yields `instantiating copy type parameter with a not implicitly copyable type` warnings. An example of such is below:

```
fn parse_command_line(args: [str]/&)
{
    let t = vec::tail(args);  // tail requires a copyable T
    io::println(#fmt["%?", t]);
}
```
