Nested operations

[Brendonovich]

With, fetch and update are implemented for non-nested actions, but going any deeper than that isn't currently possible.
Supporting this isn't hard, just takes more codegen and some more thought.

[Brendonovich]

The following should be possible:

client
    .user()
    .create(
        user::name::set("Name".to_string()),
        user::image::create(image::url::set("some url".to_string())),
    )
    .exec()
    .await;

[Brendonovich]

Nested With/Fetch Example

client
    .user()
    .find_unique(user::id::equals(0))
    .with(user::profile::fetch()
        .with(profile::image::fetch())
    )
    .exec()
    .await;

[Brendonovich]

Splitting into #45 and #44