Add Negation type

[Samasaur1]

Is your feature request related to a problem? Please describe.

It's not related to a problem, but a lack of functionality. There's no way to have a Die that can give negative results, and the only ways to do it (using Dice, CustomDie, or WeightedDie) are far too complicated to be used often.

Describe the solution you'd like:

A Negation<R: Rollable> class that negates any output from the Rollable. There should also be unary - operators for creating Negations from Rollables (and cancelling out multiple Negations.

Describe alternatives you've considered:

The class may not need to be generic; I haven't started to look at an implementation yet.

Additional context:

None

[Samasaur1]

The negation branch has some work on a non-generic Negation type (ending at 75c2e0a). It may need to be generic, so that the type returned from canceled Negations is the original type.

That said, I'm not completely sold on having a Negation struct. It feels like a makeshift patch that still leaves it very difficult to get negative numbers while still having full functionality (such as being put in Dice objects).

More consideration is required.