Inline variables printed with println! and friends (#315)

The course follows the style of inlining variable names where possible
in `println!` statements.

src/concurrency/channels/bounded.md

@@ -21,7 +21,7 @@ fn main() {
     thread::sleep(Duration::from_millis(100));
 
     for msg in rx.iter() {
-        println!("Main: got {}", msg);
+        println!("Main: got {msg}");
     }
 }
 ```

src/exercises/day-3/simple-gui.rs

@@ -24,7 +24,7 @@ pub trait Widget {
     fn draw(&self) {
         let mut buffer = String::new();
         self.draw_into(&mut buffer);
-        println!("{}", &buffer);
+        println!("{buffer}");
     }
 }
 

src/exercises/day-4/dining-philosophers.rs

@@ -90,6 +90,6 @@ fn main() {
 
     drop(tx);
     for thought in rx {
-        println!("{}", thought);
+        println!("{thought}");
     }
 }

src/generics/trait-objects.md

@@ -6,7 +6,7 @@ We've seen how a function can take arguments which implement a trait:
 use std::fmt::Display;
 
 fn print<T: Display>(x: T) {
-    println!("Your value: {}", x);
+    println!("Your value: {x}");
 }
 
 fn main() {

src/unsafe/mutable-static-variables.md

@@ -6,7 +6,7 @@ It is safe to read an immutable static variable:
 static HELLO_WORLD: &str = "Hello, world!";
 
 fn main() {
-    println!("HELLO_WORLD: {}", HELLO_WORLD);
+    println!("HELLO_WORLD: {HELLO_WORLD}");
 }
 ```
 
@@ -23,7 +23,7 @@ fn add_to_counter(inc: u32) {
 fn main() {
     add_to_counter(42);
 
-    unsafe { println!("COUNTER: {}", COUNTER); }  // Potential data race!
+    unsafe { println!("COUNTER: {COUNTER}"); }  // Potential data race!
 }
 ```