Add sugar 🍬 and spice 🧂 and everything nice ✨ to Girl, and snips ✂️ snails 🐌 and puppy-dogs' tails 🐾 to Boy

[eutro]

This PR adds some essential fields to the Girl 👧 and Boy 👦 structs, making the example a little more interesting, but still very blazingly 🔥 fast 🚀 and memory safe 🦀.

For example:

$ ./cve-rs transition girl-to-boy
Creating a Girl struct
Enter age: 100
Enter name: B. Szilvasy
Enter github username: eutro
Enter sugar and spice and everything nice values, separated by commas: beet, cinnamon, too much to write down!
Before transmute: Girl { age: 100, name: "B. Szilvasy", github_username: "eutro", sugar: 2.1497107, spice: 0.7899564, everything_nice: -0.43625119792971684 }
After transmute: Boy { age: 100, name: "B. Szilvasy", github_username: "eutro", snips: 1074369756, snails: 1061829269, puppy_dogs_tails: 13824902459702426549 }