Fix one-to-many relationship for SQLite and MySQL

[mjovanc]

We need to be able to fix the possibility of storing a vector of structs that derive the Table procedural macro:

#[derive(Table)]
#[table_name = "orders"]
pub struct Order {
    id: usize,
    user: User,             // one-to-one relationship
    products: Vec<Product>, // one-to-many relationship - populates from based on junction table (gets from macro attribute "table_name" and combines them for example, orders_products)
    total_cost: f64,
}

As you can see here when we define a field called products and add a vector of Product. It will complain about:

- doesn't satisfy `syn::Type: ToString` or `syn::Type: std::fmt::Display`

We need to be able to support this in the njord_derive library.

[mjovanc]

@chaseWillden Do you have any idea how we can solve this if we haven't already?