add #[must_use] to functions of slice and its iterators.

tests/ui/bytes_nth.fixed

@@ -5,7 +5,7 @@
 
 fn main() {
     let s = String::from("String");
-    s.as_bytes().get(3);
+    let _ = s.as_bytes().get(3);
     let _ = &s.as_bytes().get(3);
-    s[..].as_bytes().get(3);
+    let _ = s[..].as_bytes().get(3);
 }

tests/ui/bytes_nth.rs

@@ -5,7 +5,7 @@
 
 fn main() {
     let s = String::from("String");
-    s.bytes().nth(3);
+    let _ = s.bytes().nth(3);
     let _ = &s.bytes().nth(3);
-    s[..].bytes().nth(3);
+    let _ = s[..].bytes().nth(3);
 }

tests/ui/iter_next_slice.fixed

@@ -6,16 +6,16 @@ fn main() {
     let s = [1, 2, 3];
     let v = vec![1, 2, 3];
 
-    s.get(0);
+    let _ = s.get(0);
     // Should be replaced by s.get(0)
 
-    s.get(2);
+    let _ = s.get(2);
     // Should be replaced by s.get(2)
 
-    v.get(5);
+    let _ = v.get(5);
     // Should be replaced by v.get(5)
 
-    v.get(0);
+    let _ = v.get(0);
     // Should be replaced by v.get(0)
 
     let o = Some(5);

tests/ui/iter_next_slice.rs

@@ -6,16 +6,16 @@ fn main() {
     let s = [1, 2, 3];
     let v = vec![1, 2, 3];
 
-    s.iter().next();
+    let _ = s.iter().next();
     // Should be replaced by s.get(0)
 
-    s[2..].iter().next();
+    let _ = s[2..].iter().next();
     // Should be replaced by s.get(2)
 
-    v[5..].iter().next();
+    let _ = v[5..].iter().next();
     // Should be replaced by v.get(5)
 
-    v.iter().next();
+    let _ = v.iter().next();
     // Should be replaced by v.get(0)
 
     let o = Some(5);