SystemParamBuilder - 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 = (
    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 p0: DynSystemParam, mut p1: DynSystemParam, mut p2: DynSystemParam| {
            let local = p0.downcast_mut::<Local<usize>>().unwrap();
            let query_count = p1.downcast_mut::<Query<()>>().unwrap();
            let entities = p2.downcast_mut::<&Entities>().unwrap();
        },
    );

[alice-i-cecile]

This needs an end-to-end example in the form of a doc-test, probably on DynSystemParam to be properly taught to users. Handy though, and I appreciate how thorough you've been with the design and safety invariants.

[alice-i-cecile]

Nice docs!