Ownership P2: Add Closures Example

[jespiron]

This PR adds the following example to Lecture 8.
Note: I'm also open to adding this to Lecture 7 instead, and deleting the Crates section from Lecture 7

Clarifies some concepts from Lecture 7: Closures and Iterators using Ownership P2's new ownership model:

• Why moving a value out of a closure makes it FnOnce
• Why a captured value in a closure is dropped when the closure goes out of scope, not when we call and return from the closure

Also solidifies what the move keyword does, which prepares them for the Parallelism and Concurrency lectures