apply automatic clippy fixes to examples (#1022)

```sh
for ex in exercises/*; do
   (
     cd "$ex"
     cp example.rs src/lib.rs
     if [ -f Cargo-example.toml ]; then
       cp Cargo-example.toml Cargo.toml
     fi
     cargo +nightly clippy -Z unstable-options --fix --allow-dirty
     cp src/lib.rs example.rs
     git checkout -- src/lib.rs Cargo.toml
  )
done
```

Author: coriolinus

exercises/bowling/example.rs

@@ -43,7 +43,7 @@ impl Frame {
             return self.bonus_score() <= 10;
         }
 
-        if let Some(first) = self.bonus.iter().next() {
+        if let Some(first) = self.bonus.get(0) {
             if *first == 10 {
                 self.bonus_score() <= 20
             } else {

exercises/doubly-linked-list/example.rs

@@ -106,7 +106,7 @@ impl<T> LinkedList<T> {
 impl<T> Drop for LinkedList<T> {
     fn drop(&mut self) {
         let mut cursor = self.cursor_front();
-        while let Some(_) = cursor.take() {}
+        while cursor.take().is_some() {}
     }
 }
 

exercises/fizzy/example.rs

@@ -120,7 +120,7 @@ mod test {
             "fizz", "fizz", "fizz", "buzz", "buzz", "16", "8", "4", "2", "1",
         ];
         let got = fizz_buzz()
-            .apply(collatz_12.into_iter().cloned())
+            .apply(collatz_12.iter().cloned())
             .collect::<Vec<_>>();
         assert_eq!(expect, got);
     }

exercises/forth/example.rs

@@ -155,7 +155,7 @@ impl Forth {
     }
 
     fn pop_two(&mut self) -> StackResult<(Value, Value)> {
-        self.pop().and_then(|b| self.pop().and_then(|a| Ok((a, b))))
+        self.pop().and_then(|b| self.pop().map(|a| (a, b)))
     }
 
     fn bin_op<F>(&mut self, op: F) -> ForthResult

exercises/phone-number/example.rs

@@ -5,7 +5,7 @@ pub fn number(user_number: &str) -> Option<String> {
 
     if number_len < 10
         || number_len > 11
-        || (filtered_number.len() == 11 && filtered_number.chars().next().unwrap() != '1')
+        || (filtered_number.len() == 11 && !filtered_number.starts_with('1'))
     {
         return None;
     }

exercises/poker/example.rs

@@ -9,7 +9,7 @@ use counter::Counter;
 /// the winning hand(s) as were passed in, not reconstructed strings which happen to be equal.
 pub fn winning_hands<'a>(hands: &[&'a str]) -> Option<Vec<&'a str>> {
     let mut hands = hands
-        .into_iter()
+        .iter()
         .map(|source| Hand::try_from(*source))
         .collect::<Result<Vec<_>, _>>()
         .ok()?;

exercises/rail-fence-cipher/example.rs

@@ -17,13 +17,11 @@ impl RailFence {
                 *down = false;
                 *rail -= 1;
             }
+        } else if *rail > 0 {
+            *rail -= 1;
         } else {
-            if *rail > 0 {
-                *rail -= 1;
-            } else {
-                *down = true;
-                *rail += 1;
-            }
+            *down = true;
+            *rail += 1;
         }
     }