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/running-the-course.md

@@ -19,31 +19,31 @@ Before you run the course, you will want to:
    you have a clean screen to present to the class.
 
 1. Decide on the dates. Since the course takes four days, we recommend that you
-   schedule the days over two weeks. Course participants have said that
-   they find it helpful to have a gap in the course since it helps them process
-   all the information we give them.
+   schedule the days over two weeks. Course participants have said that they
+   find it helpful to have a gap in the course since it helps them process all
+   the information we give them.
 
 1. Find a room large enough for your in-person participants. We recommend a
    class size of 15-25 people. That's small enough that people are comfortable
    asking questions --- it's also small enough that one instructor will have
-   time to answer the questions. Make sure the room has _desks_ for yourself and for the
-   students: you will all need to be able to sit and work with your laptops.
-   In particular, you will be doing a lot of live-coding as an instructor, so a lectern won't
-   be very helpful for you.
+   time to answer the questions. Make sure the room has _desks_ for yourself and
+   for the students: you will all need to be able to sit and work with your
+   laptops. In particular, you will be doing a lot of live-coding as an
+   instructor, so a lectern won't be very helpful for you.
 
 1. On the day of your course, show up to the room a little early to set things
    up. We recommend presenting directly using `mdbook serve` running on your
-   laptop (see the [installation instructions][3]). This ensures optimal performance with no lag as you change pages.
-   Using your laptop will also allow you to fix typos as you or the course
-   participants spot them.
+   laptop (see the [installation instructions][3]). This ensures optimal
+   performance with no lag as you change pages. Using your laptop will also
+   allow you to fix typos as you or the course participants spot them.
 
-1. Let people solve the exercises by themselves or in small groups.
-   We typically spend 30-45 minutes on exercises in the morning and in the afternoon (including time to review the solutions).
-   Make sure to
-   ask people if they're stuck or if there is anything you can help with. When
-   you see that several people have the same problem, call it out to the class
-   and offer a solution, e.g., by showing people where to find the relevant
-   information in the standard library.
+1. Let people solve the exercises by themselves or in small groups. We typically
+   spend 30-45 minutes on exercises in the morning and in the afternoon
+   (including time to review the solutions). Make sure to ask people if they're
+   stuck or if there is anything you can help with. When you see that several
+   people have the same problem, call it out to the class and offer a solution,
+   e.g., by showing people where to find the relevant information in the
+   standard library.
 
 That is all, good luck running the course! We hope it will be as much fun for
 you as it has been for us!