Introduce pointers better

[illicitonion]

We added some pointers material to the prep section, but it wasn't enough for the last run. We should talk more about pointers (and memory) so our trainees have more of an intuition for them.

Specific goals:

• Know when to use a pointer vs value in a function signature
• Be able to reason about what changes will propagate to a caller's view vs not when a function takes a pointer vs value
• Having instincts for when adding a & or * is necessary