Add exercise opaque type aliases (#121)

* Add slide for extra exercise on opaque type aliases

* Rename exercise on opaque type aliases

- Make it apparent in the exercise name that this is an optional
  exercise

* Checkpoint result of running 'ctma renumber-exercises -f 9 -t 10 -s 1'

* Checkpoint result of running 'ctma duplicate-insert-before -n 9'

* Add exercise and instructions

- Extra exercise on opaque type aliases and comparing them to
  other alternatives such as value classes

* Checkpoint result of running 'ctma renumber-exercises -f 13 -t 20 -s 1'

Author: eloots

exercises/exercise_010_opaque_type_aliases/.scalafmt.conf renamed to exercises/exercise_010_exploring_opaque_type_aliases/.scalafmt.conf

@@ -0,0 +1,45 @@
+# Exploring Opaque type aliases
+
+## Background
+
+In the chapter on Opaque type aliases, we did a quick review of the various features in
+Scala that allow us to abstract over certain types without incurring the overhead of
+boxing/unboxing. We saw that some of these features (Value classes for example) fail
+to achieve this goal under certain circumstances.
+
+In this exercise, you will look at the aforementioned features and have a look at the
+generated byte code to see whether boxing occurs or not.
+
+## Steps
+
+- Clone this repo: [Moving from Scala 2 to Scala 3](https://github.com/lunatech-labs/lunatech-scala2-to-scala3-course)
+
+- In the cloned repository, `cd` into the `code-snippets` folder
+- This folder holds a multi-project `sbt` build. Load this project into your IDE (IntelliJ
+  or VSCode with Metals)
+- We will focus on the `opaque-type-aliases` `sbt` project for this exercise
+
+- Start an `sbt` session in the `code-snippets` folder and compile the project
+
+We will now walk through the different features
+
+### Case class wrappers
+
+- Have a look at the source code in the `CaseClasses.scala` file in the `opaquetypelaliases.caseclasses` package
+- The Scala compiler has generated class files for this under the `target/scala-3.3.0/classes/opaquetypealiases/caseclasses` folder
+- Decompile the different class files using:
+    - VSCode with Metals: the `cfr` Java decompiler: right-click on a class file. Select `Metals Analyse Source`/`Show decompiled with CFR`
+    - IntelliJ: right-click on a class file. Select `Show Decompiled Class As Java`
+- Look at the decompiled code and figure out whether boxing occurs or not.
+
+### Value-class wrappers
+
+- Repeat the same steps for file `ValueClasses.scala` in the `opaquetypelaliases.valueclasses` package
+
+- Repeat for `ParametricPolymorphism.scala` in the `opaquetypelaliases.parametricpolymorphism` package
+
+- Repeat for `Subtyping.scala` in the `opaquetypelaliases.subtyping` package
+
+### Opaque type aliases
+
+- Repeat the same steps for file `UsingTheAliases.scala` in the `opaquetypelaliases.opaquetypealias`