Add a day 5 class on unsafe Rust

Currently we briefly introduce unsafe Rust at the end of day 4, but there's a lot more to go into with unsafe Rust. I think we could have an entire day 5 course going into more detail with unsafe Rust. There's enough to cover with unsafe that we couldn't fit it all in one day, so I think we should aim for the goal of getting students ready to review unsafe code, rather than trying to get students to the point where they could write unsafe themselves.