Pattern matching: What, where, how

Basically, there are two types of pattern matching techniques that we want to cover:

1. Destructuring ADTs and lists (and especially user-defined datatypes
2. Other techniques for pattern matching, such as:
* Functions Guards
* Pattern Guards
* As-patterns
* View Patterns

The first type should be mentioned in the lesson about user-defined ADTs.
The second type will need its own lesson.

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cc @DrewFenwick