Handle &CStr <-> const char*

For a C++ function like this:

```cpp
void example(const char *s);
```

it should work to call it as:

```rust
use std::ffi::CStr;

#[cxx::bridge]
mod ffi {
    unsafe extern "C++" {
        fn example(s: &CStr);
    }
}

fn main() {
    let cstr = CStr::from_bytes_with_nul(b"test\0").unwrap();
    ffi::example(cstr);
}
```