Fix id field during instantiation of Table derived struct based on proc attribute

[mjovanc]

We need to add #[primary_key] or something similar over the field like this:

#[derive(Table)]
#[table_name = "categories"]
pub struct Category {
    #[primary_key]
    id: usize,
    name: String,
}

So when instantiating it, it should be enough to do:

let table_row: User = User {
        name: "mjovanc".to_string(),
    };

This is important since when doing INSERT operations when having autoincrement on the id field and not knowing the current id beforehand, and we definitely do not want to query to get that information before...