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/control-flow-basics/functions.md

@@ -5,6 +5,7 @@ minutes: 3
 # Functions
 
 <!-- mdbook-xgettext: skip -->
+
 ```rust,editable
 fn gcd(a: u32, b: u32) -> u32 {
     if b > 0 {
@@ -21,12 +22,18 @@ fn main() {
 
 <details>
 
-* Declaration parameters are followed by a type (the reverse of some programming languages), then a return type.
-* The last expression in a function body (or any block) becomes the return value. Simply omit the `;` at the end of the expression.
-  The `return` keyword can be used for early return, but the "bare value" form is idiomatic at the end of a function (refactor `gcd` to use a `return`).
-* Some functions have no return value, and return the 'unit type', `()`. The compiler will infer this if the `-> ()` return type is omitted.
-* Overloading is not supported -- each function has a single implementation.
-  * Always takes a fixed number of parameters. Default arguments are not supported. Macros can be used to support variadic functions.
-  * Always takes a single set of parameter types. These types can be generic, which will be covered later.
+- Declaration parameters are followed by a type (the reverse of some programming
+  languages), then a return type.
+- The last expression in a function body (or any block) becomes the return
+  value. Simply omit the `;` at the end of the expression. The `return` keyword
+  can be used for early return, but the "bare value" form is idiomatic at the
+  end of a function (refactor `gcd` to use a `return`).
+- Some functions have no return value, and return the 'unit type', `()`. The
+  compiler will infer this if the `-> ()` return type is omitted.
+- Overloading is not supported -- each function has a single implementation.
+  - Always takes a fixed number of parameters. Default arguments are not
+    supported. Macros can be used to support variadic functions.
+  - Always takes a single set of parameter types. These types can be generic,
+    which will be covered later.
 
 </details>