Improve error message on signatures containing self by value

The current error is shown as:

```console
error[cxxbridge]: unsupported signature
┌─ /dev/stdin:1:54
│
1 │ #[cxx::bridge] mod ffi { extern "C++" { type T; fn f(self); }}
│ ^^^^ unsupported signature
```

Instead it should say what to use instead. `&self` or `&mut self` for shared structs and opaque Rust types, `self: Pin<&mut Self>` for an opaque C++ type, `Box` or `UniquePtr` if ownership needs to be transferred depending on opaque Rust or opaque C++ type.