Support more general use cases for the extensible visitor pattern

[soareschen]

These are the current limitations of the extensible visitor pattern, which may prevent it from being used for more general use cases:

• The visitor dispatcher currently doesn't accept additional input arguments aside from your enum. You can workaround it by supplying it in the App context and then getting it through dependency injection.
• The HasFields implementation for enums currently lacks a version for &mut references, and thus needs to be generalized.
• Currently you can't return references in the result, due to limitations in the current implementation.
• Currently you need to manually wrap and unwrap the results via traits like ComputerRef. In the future, I plan to allow these wrappers to be automatically generated through macros, and support dispatching on custom traits through that.