Disjointed QueryData access

[vil-mo]

Requesting the QueryData implementer that allows disjointed access. Similar to ParamSet, but for QueryData.
From this point I will call such implementer DataSet (similar to how ParamSet for SystemParam was named)
Allows querying this: Query<DataSet<(&Transform, &mut Transform)>>

Possible implementation: Similar to the ParamSet implementation.
Requires cloning WorldQuery::Fetch in call to WorldQuery::fetch
Requires adding method to the WorldQuery trait similar to WorldQuery::shrink, but for WorldQuery::Fetch as input and output so that it is possible to implement WorldQuery::shrink

pub struct DataSet<'w, T: QueryData> {
    fetch: T::Fetch<'w>,
    entity: Entity,
    table_row: TableRow,
}

I have made basic proof-of-concept implementation without proc macro just for 2 parameters in set: https://github.com/vil-mo/bevy_data_set

[vil-mo]

Will be useful in generic contexts, for example

trait QueryDataFunction {
    type Data: QueryData;
    type Out;

    fn run(data: Self::Data) -> Out;
}

/// System that runs B only if A returns true
fn run_if_true<
    A: QueryDataFunction<Out = bool>,
    B: QueryDataFunction<Out = ()>,
>(Query<DataSet<(A::Data, B::Data)>>) {
    for mut set in query.iter_mut() {
        if A::run(set.d0()) {
            B::run(set.d1());
        }
    }
}