Format all Markdown files with dprint (#1157)

This is the result of running `dprint fmt` after removing `src/` from
the list of excluded directories.

This also reformats the Rust code: we might want to tweak this a bit in
the future since some of the changes removes the hand-formatting. Of
course, this formatting can be seen as a mis-feature, so maybe this is
good overall.

Thanks to mdbook-i18n-helpers 0.2, the POT file is nearly unchanged
after this, meaning that all existing translations remain valid! A few
messages were changed because of stray whitespace characters:

     msgid ""
     "Slices always borrow from another object. In this example, `a` has to remain "
    -"'alive' (in scope) for at least as long as our slice. "
    +"'alive' (in scope) for at least as long as our slice."
     msgstr ""

The formatting is enforced in CI and we will have to see how annoying
this is in practice for the many contributors. If it becomes annoying,
we should look into fixing dprint/check#11 so that `dprint` can annotate
the lines that need fixing directly, then I think we can consider more
strict formatting checks.

I added more customization to `rustfmt.toml`. This is to better emulate
the dense style used in the course:

- `max_width = 85` allows lines to take up the full width available in
our code blocks (when taking margins and the line numbers into account).
- `wrap_comments = true` ensures that we don't show very long comments
in the code examples. I edited some comments to shorten them and avoid
unnecessary line breaks — please trim other unnecessarily long comments
when you see them! Remember we're writing code for slides 😄
- `use_small_heuristics = "Max"` allows for things like struct literals
and if-statements to take up the full line width configured above.

The formatting settings apply to all our Rust code right now — I think
we could improve this with https://github.com/dprint/dprint/issues/711
which lets us add per-directory `dprint` configuration files. However,
the `inherit: true` setting is not yet implemented (as far as I can
tell), so a nested configuration file will have to copy most or all of
the top-level file.

src/methods-and-traits/methods.md

@@ -15,22 +15,26 @@ struct Race {
 }
 
 impl Race {
-    fn new(name: &str) -> Self {  // No receiver, a static method
+    // No receiver, a static method
+    fn new(name: &str) -> Self {
         Self { name: String::from(name), laps: Vec::new() }
     }
 
-    fn add_lap(&mut self, lap: i32) {  // Exclusive borrowed read-write access to self
+    // Exclusive borrowed read-write access to self
+    fn add_lap(&mut self, lap: i32) {
         self.laps.push(lap);
     }
 
-    fn print_laps(&self) {  // Shared and read-only borrowed access to self
+    // Shared and read-only borrowed access to self
+    fn print_laps(&self) {
         println!("Recorded {} laps for {}:", self.laps.len(), self.name);
         for (idx, lap) in self.laps.iter().enumerate() {
             println!("Lap {idx}: {lap} sec");
         }
     }
 
-    fn finish(self) {  // Exclusive ownership of self
+    // Exclusive ownership of self
+    fn finish(self) {
         let total: i32 = self.laps.iter().sum();
         println!("Race {} is finished, total lap time: {}", self.name, total);
     }
@@ -48,32 +52,42 @@ fn main() {
 }
 ```
 
-The `self` arguments specify the "receiver" - the object the method acts on. There
-are several common receivers for a method:
+The `self` arguments specify the "receiver" - the object the method acts on.
+There are several common receivers for a method:
 
-* `&self`: borrows the object from the caller using a shared and immutable
+- `&self`: borrows the object from the caller using a shared and immutable
   reference. The object can be used again afterwards.
-* `&mut self`: borrows the object from the caller using a unique and mutable
+- `&mut self`: borrows the object from the caller using a unique and mutable
   reference. The object can be used again afterwards.
-* `self`: takes ownership of the object and moves it away from the caller. The
-  method becomes the owner of the object. The object will be dropped (deallocated)
-  when the method returns, unless its ownership is explicitly
+- `self`: takes ownership of the object and moves it away from the caller. The
+  method becomes the owner of the object. The object will be dropped
+  (deallocated) when the method returns, unless its ownership is explicitly
   transmitted. Complete ownership does not automatically mean mutability.
-* `mut self`: same as above, but the method can mutate the object.
-* No receiver: this becomes a static method on the struct. Typically used to
+- `mut self`: same as above, but the method can mutate the object.
+- No receiver: this becomes a static method on the struct. Typically used to
   create constructors which are called `new` by convention.
 
 <details>
 
 Key Points:
-* It can be helpful to introduce methods by comparing them to functions.
-  * Methods are called on an instance of a type (such as a struct or enum), the first parameter represents the instance as `self`.
-  * Developers may choose to use methods to take advantage of method receiver syntax and to help keep them more organized. By using methods we can keep all the implementation code in one predictable place.
-* Point out the use of the keyword `self`, a method receiver.
-  * Show that it is an abbreviated term for `self: Self` and perhaps show how the struct name could also be used.
-  * Explain that `Self` is a type alias for the type the `impl` block is in and can be used elsewhere in the block.
-  * Note how `self` is used like other structs and dot notation can be used to refer to individual fields.
-  * This might be a good time to demonstrate how the `&self` differs from `self` by trying to run `finish` twice.
-  * Beyond variants on `self`, there are also [special wrapper types](https://doc.rust-lang.org/reference/special-types-and-traits.html) allowed to be receiver types, such as `Box<Self>`.
+
+- It can be helpful to introduce methods by comparing them to functions.
+  - Methods are called on an instance of a type (such as a struct or enum), the
+    first parameter represents the instance as `self`.
+  - Developers may choose to use methods to take advantage of method receiver
+    syntax and to help keep them more organized. By using methods we can keep
+    all the implementation code in one predictable place.
+- Point out the use of the keyword `self`, a method receiver.
+  - Show that it is an abbreviated term for `self: Self` and perhaps show how
+    the struct name could also be used.
+  - Explain that `Self` is a type alias for the type the `impl` block is in and
+    can be used elsewhere in the block.
+  - Note how `self` is used like other structs and dot notation can be used to
+    refer to individual fields.
+  - This might be a good time to demonstrate how the `&self` differs from `self`
+    by trying to run `finish` twice.
+  - Beyond variants on `self`, there are also
+    [special wrapper types](https://doc.rust-lang.org/reference/special-types-and-traits.html)
+    allowed to be receiver types, such as `Box<Self>`.
 
 </details>