Rework iterator section

[randomPoison]

This PR attempts to improve clarity and flow in the section on iterators. I don't think the current materials do a good job of motivating iterators or putting them in a context where students can understand them. This PR does the following to try to improve this:

• Add motivation slide. This slide uses the example of using a C-style for loop to iterate over the contents of an array. This helps contextualize the concept of iteration in a way that most students will understand; Many languages have C-style for loops, so students are likely going to be familiar with the syntax and will have seen for (i = 0; i < len; i++) many times.
• Replace fibonacci example with slice iter example. This continues the idea from the previous slide, demonstrating how to implement the same concept (iterating over the contents of an array) using Iterator. I also think this example is easier to understand than the fibonacci one, I found the logic there kind of confusing/hard to explain to students. It also demonstrates an iterator with a termination case, whereas the fibonacci example never terminates.
• Add slide for iterator helper methods. I think splitting helper methods into its own slide gives them a bit more room to breathe. This also allows the previous slide to be more focused on implementing Iterator.
• Reorder slides to talk about collect before IntoIterator. I think this flows better after the slide on helper methods since this is just calling out a specific helper method.
• Add speaker notes and reword things for clarity/focus.

[randomPoison]

As I put this PR up it occurs to me that we don't need to use C-style for to demonstrate this concept, we could show the same thing in pure Rust:

let mut i = 0;
while i < array.len() {
    let elem = array[i];
    i += 1;
}

Might still be good to mention C-style for in the speaker notes, since it's still a common point of reference.

[randomPoison]

I kinda still feel like the C version is going to be clearer and more familiar for most students, tho

[randomPoison]

@djmitche Do you have an opinion here? I'm leaning towards leaving this as a c-style for for readability and familiarity

[randomPoison]

You know what, I'm going to leave it as a C-style for but I'm going to add the rust equivalent in the speaker notes for easy reference. Let me know if you think that's a bad idea for whatever reason, though.

[djmitche]

I think the C-style is fine.

[djmitche]

This is a great improvement to this segment!

[djmitche]

Minor detail, but I'd like to get away from the idea that all lifetimes are named 'a -- maybe use 'sl instead?

[randomPoison]

How about 's?

[djmitche]

's is good!