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Show expressions trees (#2425)

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While giving the class, I noticed that a few people were not used to
this kind of recursive structures. A diagram could help here.
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@ -6,6 +6,52 @@ minutes: 30
Let's write a simple recursive evaluator for arithmetic expressions.
An example of a small arithmetic expression could be `10 + 20`, which evaluates
to `30`. We can represent the expression as a tree:
<!-- mdbook-xgettext: skip -->
```bob
.-------.
.------ | + | ------.
| '-------' |
v v
.--------. .--------.
| 10 | | 20 |
'--------' '--------'
```
A bigger and more complex expression would be `(10 * 9) + ((3 - 4) * 5)`, which
evaluate to `85`. We represent this as a much bigger tree:
<!-- mdbook-xgettext: skip -->
```bob
.-----.
.---------------- | + | ----------------.
| '-----' |
v v
.-----. .-----.
.---- | * | ----. .---- | * | ----.
| '-----' | | '-----' |
v v v v
.------. .-----. .-----. .-----.
| 10 | | 9 | .---- | "-"| ----. | 5 |
'------' '-----' | '-----' | '-----'
v v
.-----. .-----.
| 3 | | 4 |
'-----' '-----'
```
In code, we will represent the tree with two types:
```rust
{{#include exercise.rs:Operation}}
{{#include exercise.rs:Expression}}
```
The `Box` type here is a smart pointer, and will be covered in detail later in
the course. An expression can be "boxed" with `Box::new` as seen in the tests.
To evaluate a boxed expression, use the deref operator (`*`) to "unbox" it: