Make it easier to iterate through DiceKit types

[Samasaur1]

One example of this problem is here. Take a look at the doc comment for the chances property of a Chances object.

DiceKit/Sources/DiceKit/Chances.swift

Lines 31 to 46 in d67f52f

	/// The rolls and the chances of them occurring.
	///
	/// This is the property one should use in order to iterate through the possibilities, like so:
	///
	/// ```
	/// let chances: Chances = getChancesFromSomewhereElse()
	/// let arr = chances.chances.sorted(by: { first, second in
	/// first.key < second.key
	/// })
	/// for (roll, chance) in arr {
	/// print("The chance of rolling a \(roll) is \(chance.n) out of \(chance.d)")
	/// }
	/// ```
	///
	/// - Since: 0.21.0
	public private(set) var chances: [Roll: Chance]

It wants you to use chances.chances, and then possibly sorted(by:) on top of that, if you want to iterate through it. However, Swift has support for adding iteration to custom types, using IteratorProtocol. While I'm not sure how far we should take this idea (mapping? conformance to Collection? other DiceKit type?), these two ideas seems like a good start:

import DiceKit

let dice: Dice = getDiceStruct()
for die in dice {
    print(die)
}
for (die, count) in dice {
    print("Die \(die) is repeated \(count) time(s)")
}

let chances: Chances = getChances()
for (roll, chance) in chances {
    print("The probability of rolling a \(roll) is \(chance)")
}

I'm not as sure about iterating through Dice, in part because I do see two ways to do that, as I listed above. Given 2d6 + d4, the first one would loop three times (d6, d6, d4), and the second would loop twice ((d6, 2), (d4, 1)). More consideration is required on that front. However, the idea overall seems sound.

[Samasaur1]

Well actually, the user might want to iterate through Chances in order of likelihood. The implementation of IteratorProtocol in #96 is not the only option.

What might be a step in the right direction would be to allow calling sorted(by:) directly on the Chances object, rather than on Chances.chances (the repetition is confusing). This would turn this:

let chances = getChances()
for (roll, chance) in chances.chances.sorted(by: { $0.key < $1.key }) {
    print("The chance of rolling a \(roll) is \(chance.n) out of \(chance.d)")
}

(which is already condensed) into this:

let chances = getChances()
for (roll, chance) in chances.sorted(by: { $0.key < $1.key }) {
    print("The chance of rolling a \(roll) is \(chance.n) out of \(chance.d)")
}

It's not perfect, but it's definitely an improvement, and feels more similar to Dictionary