Support dynamic system parameters.

[chescock]

Objective

Support building systems with parameters whose types can be determined at runtime.

Solution

Create a DynSystemParam type that can be built using a SystemParamBuilder of any type and then downcast to the appropriate type dynamically.

Example

let system = (vec![
    DynParamBuilder::new(LocalBuilder(3_usize)),
    DynParamBuilder::new::<Query<()>>(QueryParamBuilder::new(|builder| {
        builder.with::<A>();
    })),
    DynParamBuilder::new::<&Entities>(ParamBuilder),
],)
    .build_state(&mut world)
    .build_system(|mut params: Vec<DynSystemParam>| {
        let local = *params[0].downcast_mut::<Local<usize>>().unwrap();
        let query_count = params[1]
            .downcast_mut::<Query<()>>()
            .unwrap()
            .iter()
            .count();
        let entities = params[2].downcast_mut::<&Entities>().unwrap();
    });