Merge recent update from ch7 && cargo clippy

user/src/bin/cmdline_args.rs

@@ -9,8 +9,8 @@ extern crate user_lib;
 #[no_mangle]
 pub fn main(argc: usize, argv: &[&str]) -> i32 {
     println!("argc = {}", argc);
-    for i in 0..argc {
-        println!("argv[{}] = {}", i, argv[i]);
+    for (i, arg) in argv.iter().enumerate() {
+        println!("argv[{}] = {}", i, arg);
     }
     0
-}
+}