impl `Copy` for `NoSend`/`NoSync`

Necessary to implement `Copy` on structs like this one:

``` rust
struct Slice<'a, T> {
    _contravariant: marker::ContravariantLifetime<'a>,
    _nosend: marker::NoSend,
    data: *const T,
    length: uint,
}
```

r? @alexcrichton