* Update tuple-structs.md
I am adding more information about newtypes. The speaker can get questions about it or may wish to encourage this great pattern.
* Fix typos and explanation
Co-authored-by: Andrew Walbran <qwandor@google.com>
Introducing `Self` type that students can encounter in many places and nice to use. It is only mentioned later in the course, but it is not defined currently.