Option<I> SystemInput

[ItsDoot]

Objective

• Provide more standard system input types/combinators.

Currently, if users want a system to take Option<&mut T> or Option<&T> as system input, they need to either use InMut<Option<T>>/InRef<Option<T>> which actually result in a &mut Option<T>/&Option<T>, or create their own SystemInput type.

Solution

Implement SystemInput for Option<I> where I can be any SystemInput type.

Testing

Added a test for this.

[alice-i-cecile]

I'd prefer World::run_system_once in tests, but that's just a style nit :)