Use implicit capture syntax in format_args

This updates the standard library's documentation to use the new syntax. The
documentation is worthwhile to update as it should be more idiomatic
(particularly for features like this, which are nice for users to get acquainted
with). The general codebase is likely more hassle than benefit to update: it'll
hurt git blame, and generally updates can be done by folks updating the code if
(and when) that makes things more readable with the new format.

A few places in the compiler and library code are updated (mostly just due to
already having been done when this commit was first authored).

library/std/src/primitive_docs.rs

@@ -607,7 +607,7 @@ mod prim_pointer {}
 ///
 /// // This loop prints: 0 1 2
 /// for x in array {
-///     print!("{} ", x);
+///     print!("{x} ");
 /// }
 /// ```
 ///
@@ -646,19 +646,19 @@ mod prim_pointer {}
 /// // This creates a slice iterator, producing references to each value.
 /// for item in array.into_iter().enumerate() {
 ///     let (i, x): (usize, &i32) = item;
-///     println!("array[{}] = {}", i, x);
+///     println!("array[{i}] = {x}");
 /// }
 ///
 /// // The `array_into_iter` lint suggests this change for future compatibility:
 /// for item in array.iter().enumerate() {
 ///     let (i, x): (usize, &i32) = item;
-///     println!("array[{}] = {}", i, x);
+///     println!("array[{i}] = {x}");
 /// }
 ///
 /// // You can explicitly iterate an array by value using `IntoIterator::into_iter`
 /// for item in IntoIterator::into_iter(array).enumerate() {
 ///     let (i, x): (usize, i32) = item;
-///     println!("array[{}] = {}", i, x);
+///     println!("array[{i}] = {x}");
 /// }
 /// ```
 ///
@@ -673,13 +673,13 @@ mod prim_pointer {}
 /// // This iterates by reference:
 /// for item in array.iter().enumerate() {
 ///     let (i, x): (usize, &i32) = item;
-///     println!("array[{}] = {}", i, x);
+///     println!("array[{i}] = {x}");
 /// }
 ///
 /// // This iterates by value:
 /// for item in array.into_iter().enumerate() {
 ///     let (i, x): (usize, i32) = item;
-///     println!("array[{}] = {}", i, x);
+///     println!("array[{i}] = {x}");
 /// }
 /// ```
 ///
@@ -702,26 +702,26 @@ mod prim_pointer {}
 /// // This iterates by reference:
 /// for item in array.iter() {
 ///     let x: &i32 = item;
-///     println!("{}", x);
+///     println!("{x}");
 /// }
 ///
 /// // This iterates by value:
 /// for item in IntoIterator::into_iter(array) {
 ///     let x: i32 = item;
-///     println!("{}", x);
+///     println!("{x}");
 /// }
 ///
 /// // This iterates by value:
 /// for item in array {
 ///     let x: i32 = item;
-///     println!("{}", x);
+///     println!("{x}");
 /// }
 ///
 /// // IntoIter can also start a chain.
 /// // This iterates by value:
 /// for item in IntoIterator::into_iter(array).enumerate() {
 ///     let (i, x): (usize, i32) = item;
-///     println!("array[{}] = {}", i, x);
+///     println!("array[{i}] = {x}");
 /// }
 /// ```
 ///