Constrain cyclic associated types to themselves

[BadBastion]

The Database trait's associated types are constrained using Connection<Database = Self> but this constraint is not specified for the reverse Database<Connection = Self>. This causes issues when wrapping or "extending" sqlx traits since as far as rust's type system is concerned Self::Connection::Database::Connection != Self::Connection.

Consider the following example:

type DatabaseTypeOf<T> = <<T as ConnectOptions>::Connection as Connection>::Database;

pub trait OptionWrapper {
    type Pool: Pool;
    async fn connect(self) -> Self::Pool;
}

impl <T> OptionWrapper for T
where
    T: ConnectOptions,
    for<'a> &'a mut T::Connection: Executor<'a, Database = DatabaseTypeOf<T>>,
{
    type Client = Pool<DatabaseTypeOf<T>>;

    async fn connect(self) -> Self::Pool {
        Pool::connect_with(self).await.unwrap()
        //    ^^^^^^^^^^^^^^^^^^ expected associated type, found type parameter `T`
        // type mismatch resolving `<<<<T as ConnectOptions>::Connection as Connection>::Database as Database>::Connection as Connection>::Options == T`
    }
}

---

This PR adds bounds such that cyclic associated types are equal to themselves.

<T as Connection>::Database as Database>::Connection == T
<T as ConnectOptions>::Connection as Connection>::Options == T
<T as Row>::Database as Database>::Row == T
<T as Column>::Database as Database>::Column == T
<T as Value>::Database as Database>::Value == T

[jplatte]

I guess technically this is a breaking change, but I agree that it makes a lot of sense. 👍🏼