Rework generic fn examples to show monomorphized versions

[randomPoison]

Something I always do when covering generic fns is I like to show the monomorphized versions of pick to make it clear to students what generics are doing behind the scenes. In my most recent class I tried going the other way around, showing the monomorphized versions first to more clearly motivate what generics are used for, and I liked the way it went. I think motivating generics by first showing code duplication and then showing how generics allow us to de-duplicate makes for a good teaching flow, and I think it also helps make things clearer to students coming from more dynamic languages that don't have an equivalent to generics.

I also changed the pick fns to take a bool as the first argument because I think that makes things slightly clearer/cleaner, but I'm not married to that change either.

[djmitche]

I like the change, but it probably deserves a speaker note regarding how to use the slide.

[mgeisler]

The idea of showing the result of monomorphization is great, but I'm afraid the slide become too tall now?

I would suggest putting this into the speaker notes instead: keep the pick<T> function in the slide and let the speaker notes explain that it expands to pick_i32 and pick_char depending on how it's called.

[randomPoison]

Both of your comments make sense to me, so I've moved the monomorphized versions into the speaker notes and added a note explaining that they can be used to both motivate the generic pick and demonstrate monomorphization. Let me know how it looks!