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/basic-syntax/compound-types.md

@@ -7,6 +7,7 @@
 
 Array assignment and access:
 
+<!-- mdbook-xgettext: skip -->
 ```rust,editable
 fn main() {
     let mut a: [i8; 10] = [42; 10];
@@ -17,11 +18,12 @@ fn main() {
 
 Tuple assignment and access:
 
+<!-- mdbook-xgettext: skip -->
 ```rust,editable
 fn main() {
     let t: (i8, bool) = (7, true);
-    println!("1st index: {}", t.0);
-    println!("2nd index: {}", t.1);
+    println!("t.0: {}", t.0);
+    println!("t.1: {}", t.1);
 }
 ```
 

src/basic-syntax/functions.md

@@ -2,6 +2,7 @@
 
 A Rust version of the famous [FizzBuzz](https://en.wikipedia.org/wiki/Fizz_buzz) interview question:
 
+<!-- mdbook-xgettext: skip -->
 ```rust,editable
 fn main() {
     print_fizzbuzz_to(20);

src/basic-syntax/references-dangling.md

@@ -2,6 +2,7 @@
 
 Rust will statically forbid dangling references:
 
+<!-- mdbook-xgettext: skip -->
 ```rust,editable,compile_fail
 fn main() {
     let ref_x: &i32;

src/basic-syntax/references.md

@@ -2,6 +2,7 @@
 
 Like C++, Rust has references:
 
+<!-- mdbook-xgettext: skip -->
 ```rust,editable
 fn main() {
     let mut x: i32 = 10;

src/basic-syntax/scalar-types.md

@@ -24,6 +24,7 @@ There are a few syntaxes which are not shown above:
   == "\\\\n"`. You can embed double-quotes by using an equal amount of `#` on
   either side of the quotes:
 
+  <!-- mdbook-xgettext: skip -->
   ```rust,editable
   fn main() {
       println!(r#"<a href="link.html">link</a>"#);
@@ -33,6 +34,7 @@ There are a few syntaxes which are not shown above:
 
 - Byte strings allow you to create a `&[u8]` value directly:
 
+  <!-- mdbook-xgettext: skip -->
   ```rust,editable
   fn main() {
       println!("{:?}", b"abc");

src/basic-syntax/scopes-shadowing.md

@@ -27,6 +27,7 @@ fn main() {
 * Shadowing looks obscure at first, but is convenient for holding on to values after `.unwrap()`.
 * The following code demonstrates why the compiler can't simply reuse memory locations when shadowing an immutable variable in a scope, even if the type does not change.
 
+<!-- mdbook-xgettext: skip -->
 ```rust,editable
 fn main() {
     let a = 1;

src/basic-syntax/slices.md

@@ -2,6 +2,7 @@
 
 A slice gives you a view into a larger collection:
 
+<!-- mdbook-xgettext: skip -->
 ```rust,editable
 fn main() {
     let mut a: [i32; 6] = [10, 20, 30, 40, 50, 60];

src/basic-syntax/static-and-const.md

@@ -8,6 +8,7 @@ cannot be moved or reallocated during the execution of the program.
 Constant variables are evaluated at compile time and their values are inlined
 wherever they are used:
 
+<!-- mdbook-xgettext: skip -->
 ```rust,editable
 const DIGEST_SIZE: usize = 3;
 const ZERO: Option<u8> = Some(42);
@@ -22,7 +23,7 @@ fn compute_digest(text: &str) -> [u8; DIGEST_SIZE] {
 
 fn main() {
     let digest = compute_digest("Hello");
-    println!("Digest: {digest:?}");
+    println!("digest: {digest:?}");
 }
 ```
 

src/basic-syntax/type-inference.md

@@ -2,6 +2,7 @@
 
 Rust will look at how the variable is _used_ to determine the type:
 
+<!-- mdbook-xgettext: skip -->
 ```rust,editable
 fn takes_u32(x: u32) {
     println!("u32: {x}");
@@ -31,6 +32,7 @@ The compiler does the job for us and helps us write more concise code.
 
 The following code tells the compiler to copy into a certain generic container without the code ever explicitly specifying the contained type, using `_` as a placeholder:
 
+<!-- mdbook-xgettext: skip -->
 ```rust,editable
 fn main() {
     let mut v = Vec::new();

src/basic-syntax/variables.md

@@ -3,6 +3,7 @@
 Rust provides type safety via static typing. Variable bindings are immutable by
 default:
 
+<!-- mdbook-xgettext: skip -->
 ```rust,editable
 fn main() {
     let x: i32 = 10;