Skip some untranslatable code blocks (#1258)

This builds on the work of @dyoo in
https://github.com/google/mdbook-i18n-helpers/pull/69: by adding a
special `<!-- mdbook-xgettext: skip -->` comment, we can skip the
following code block.

I also modified a few code blocks to remove translatable text: variable
names are not expected to be translated, so it’s fine to have a line
with `println!("foo: {foo}")` in the code block.

This PR removes 36 messages from the POT file. The number of lines drop
by 633 (3%).

Part of #1257.

src/exercises/day-1/for-loops.md

@@ -8,6 +8,7 @@ let array = [10, 20, 30];
 
 You can print such an array by asking for its debug representation with `{:?}`:
 
+<!-- mdbook-xgettext: skip -->
 ```rust,editable
 fn main() {
     let array = [10, 20, 30];
@@ -38,6 +39,7 @@ fn main() {
 Use the above to write a function `pretty_print` which pretty-print a matrix and
 a function `transpose` which will transpose a matrix (turn rows into columns):
 
+<!-- mdbook-xgettext: skip -->
 ```bob
            ⎛⎡1 2 3⎤⎞      ⎡1 4 7⎤
 "transpose"⎜⎢4 5 6⎥⎟  "=="⎢2 5 8⎥

src/exercises/day-1/implicit-conversions.md

@@ -3,6 +3,7 @@
 Rust will not automatically apply _implicit conversions_ between types ([unlike
 C++][3]). You can see this in a program like this:
 
+<!-- mdbook-xgettext: skip -->
 ```rust,editable,compile_fail
 fn multiply(x: i16, y: i16) -> i16 {
     x * y