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/modules/filesystem.md

@@ -11,12 +11,13 @@ mod garden;
 ```
 
 This tells rust that the `garden` module content is found at `src/garden.rs`.
-Similarly, a `garden::vegetables` module can be found at `src/garden/vegetables.rs`.
+Similarly, a `garden::vegetables` module can be found at
+`src/garden/vegetables.rs`.
 
 The `crate` root is in:
 
-* `src/lib.rs` (for a library crate)
-* `src/main.rs` (for a binary crate)
+- `src/lib.rs` (for a library crate)
+- `src/main.rs` (for a binary crate)
 
 Modules defined in files can be documented, too, using "inner doc comments".
 These document the item that contains them -- in this case, a module.
@@ -26,24 +27,29 @@ These document the item that contains them -- in this case, a module.
 //! implementation.
 
 // Re-export types from this module.
-pub use seeds::SeedPacket;
 pub use garden::Garden;
+pub use seeds::SeedPacket;
 
 /// Sow the given seed packets.
-pub fn sow(seeds: Vec<SeedPacket>) { todo!() }
+pub fn sow(seeds: Vec<SeedPacket>) {
+    todo!()
+}
 
 /// Harvest the produce in the garden that is ready.
-pub fn harvest(garden: &mut Garden) { todo!() }
+pub fn harvest(garden: &mut Garden) {
+    todo!()
+}
 ```
 
 <details>
 
-* Before Rust 2018, modules needed to be located at `module/mod.rs` instead of `module.rs`, and this is still a working alternative for editions after 2018.
+- Before Rust 2018, modules needed to be located at `module/mod.rs` instead of
+  `module.rs`, and this is still a working alternative for editions after 2018.
 
-* The main reason to introduce `filename.rs` as alternative to `filename/mod.rs`
+- The main reason to introduce `filename.rs` as alternative to `filename/mod.rs`
   was because many files named `mod.rs` can be hard to distinguish in IDEs.
 
-* Deeper nesting can use folders, even if the main module is a file:
+- Deeper nesting can use folders, even if the main module is a file:
 
   ```ignore
   src/
@@ -53,14 +59,14 @@ pub fn harvest(garden: &mut Garden) { todo!() }
       └── sub_module.rs
   ```
 
-* The place rust will look for modules can be changed with a compiler directive:
+- The place rust will look for modules can be changed with a compiler directive:
 
   ```rust,ignore
   #[path = "some/path.rs"]
   mod some_module;
   ```
 
-  This is useful, for example, if you would like to place tests for a module in a file named
-  `some_module_test.rs`, similar to the convention in Go.
+  This is useful, for example, if you would like to place tests for a module in
+  a file named `some_module_test.rs`, similar to the convention in Go.
 
 </details>