This breaks out a new QueryFilter type from HecsQuery. HecsQueries like Changed<T>, With<T>, and Without<T> have become QueryFilters. The new Query interface now looks like this: Query<Q: HecsQuery, F: QueryFilter = ()>

Ex:

Query<&A> -> Query<&A> (no change)
Query<With<X, With<Y, (&A, &mut B)>>> -> Query<(&A, &mut B), (With<X>, With<Y>)>
Query<Changed<A>> -> Query<&A, Changed<A>>

All direct world query functions now have "filtered" variants. Ex: world.query<Q: HecsQuery> and world.query_filtered<Q: HecsQuery, F: QueryFilter> variants.

This resolves a number of issues:

• Queries like With<X, With<Y, (&A, &mut B)>> are hard to visually parse
• It is currently a bit unclear what components will be returned by a query
• You can't filter "change state" without also grabbing the component
• There is no good way to get a mutable reference to a Changed (it currently returns a read-only reference). Note that there is a pr out (Make Changed, Mutated and Added work on Queries #625) to resolve this by turning Changed into Changed<&T> and Changed<&mut T>, which we would probably merge if we dont go with this pr.
• Each new "component filter" requires a new "smart pointer" type

This is how a "real" system changes:

Before

pub fn flex_node_system(
    windows: Res<Windows>,
    mut flex_surface: ResMut<FlexSurface>,
    root_node_query: Query<With<Node, Without<Parent, Entity>>>,
    node_query: Query<With<Node, (Entity, Changed<Style>, Option<&CalculatedSize>)>>,
    changed_size_query: Query<With<Node, (Entity, &Style, Changed<CalculatedSize>)>>,
    children_query: Query<With<Node, (Entity, Changed<Children>)>>,
    mut node_transform_query: Query<(Entity, &mut Node, &mut Transform, Option<&Parent>)>,
) {
}

After

pub fn flex_node_system(
    windows: Res<Windows>,
    mut flex_surface: ResMut<FlexSurface>,
    root_node_query: Query<Entity, (With<Node>, Without<Parent>)>,
    node_query: Query<(Entity, &Style, Option<&CalculatedSize>), (With<Node>, Changed<Style>)>,
    changed_size_query: Query<
        (Entity, &Style, &CalculatedSize),
        (With<Node>, Changed<CalculatedSize>),
    >,
    children_query: Query<(Entity, &Children), (With<Node>, Changed<Children>)>,
    mut node_transform_query: Query<(Entity, &mut Node, &mut Transform, Option<&Parent>)>,
) {
}