Rollup merge of #131783 - practicalrs:fix_explicit_iter_loop, r=compiler-errors

Fix explicit_iter_loop in rustc_serialize

Hi,

This PR fixes some clippy warnings

```
warning: it is more concise to loop over references to containers instead of using explicit iteration methods
   --> compiler/rustc_serialize/src/serialize.rs:675:18
    |
675 |         for e in self.iter() {
    |                  ^^^^^^^^^^^ help: to write this more concisely, try: `self`
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#explicit_iter_loop

```

Best regards,
Michal

compiler/rustc_serialize/src/serialize.rs

@@ -288,7 +288,7 @@ impl<D: Decoder, T: Decodable<D>> Decodable<D> for Rc<T> {
 impl<S: Encoder, T: Encodable<S>> Encodable<S> for [T] {
     default fn encode(&self, s: &mut S) {
         s.emit_usize(self.len());
-        for e in self.iter() {
+        for e in self {
             e.encode(s);
         }
     }
@@ -527,7 +527,7 @@ impl<D: Decoder, T: Decodable<D>> Decodable<D> for ThinVec<T> {
 impl<S: Encoder, T: Encodable<S>> Encodable<S> for VecDeque<T> {
     fn encode(&self, s: &mut S) {
         s.emit_usize(self.len());
-        for e in self.iter() {
+        for e in self {
             e.encode(s);
         }
     }
@@ -547,7 +547,7 @@ where
 {
     fn encode(&self, e: &mut S) {
         e.emit_usize(self.len());
-        for (key, val) in self.iter() {
+        for (key, val) in self {
             key.encode(e);
             val.encode(e);
         }
@@ -571,7 +571,7 @@ where
 {
     fn encode(&self, s: &mut S) {
         s.emit_usize(self.len());
-        for e in self.iter() {
+        for e in self {
             e.encode(s);
         }
     }
@@ -595,7 +595,7 @@ where
 {
     fn encode(&self, e: &mut E) {
         e.emit_usize(self.len());
-        for (key, val) in self.iter() {
+        for (key, val) in self {
             key.encode(e);
             val.encode(e);
         }
@@ -621,7 +621,7 @@ where
 {
     fn encode(&self, s: &mut E) {
         s.emit_usize(self.len());
-        for e in self.iter() {
+        for e in self {
             e.encode(s);
         }
     }
@@ -646,7 +646,7 @@ where
 {
     fn encode(&self, e: &mut E) {
         e.emit_usize(self.len());
-        for (key, val) in self.iter() {
+        for (key, val) in self {
             key.encode(e);
             val.encode(e);
         }
@@ -672,7 +672,7 @@ where
 {
     fn encode(&self, s: &mut E) {
         s.emit_usize(self.len());
-        for e in self.iter() {
+        for e in self {
             e.encode(s);
         }
     }