Get an `Id<T>` without going through toasty

Currently there is no way create an `Id<T>` without going through toasty. By for example creating, updating or selecting the model.

E.g. if you have a `user` table with an `id INT PRIMARY KEY` field, it's not possible to do
```rust
User::get_by_id(&db, 1i32)
```

You have to get an `Id<User>` somehow.