Setting individual chances in Chances has inconsistent behavior

[Samasaur1]

Here's the getter/setter for what I'm talking about.

DiceKit/Sources/DiceKit/Chances.swift

Lines 52 to 59 in d67f52f

	public subscript(of roll: Roll) -> Chance {
	get {
	return chances[roll] ?? Chance.zero
	}
	set {
	chances[roll] = newValue
	}
	}

As you can see, when getting a Chance, the assumption is that if it is not there, it's zero. That's what we want. However, when we set a chance to zero, we add it to the dictionary, potentially resulting in a large number of entries with Chance.zero as their values. This is unnecessary, could potentially be detrimental (storage space), and is internally inconsistent. I think it doesn't actually have an outward-facing effect, because we filter out chances that are not greater than zero:

DiceKit/Sources/DiceKit/Chances.swift

Line 27 in d67f52f

for (roll, chance) in chances where chance.n > 0 {

but we could and probably should still change this.