Resolve explicit_iter_loop pedantic clippy lint

    error: it is more concise to loop over references to containers instead of using explicit iteration methods
        --> serde/src/private/de.rs:2761:22
         |
    2761 |         for entry in self.0.iter_mut() {
         |                      ^^^^^^^^^^^^^^^^^ help: to write this more concisely, try: `&mut *self.0`
         |
         = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#explicit_iter_loop
         = note: `-D clippy::explicit-iter-loop` implied by `-D clippy::pedantic`

    error: it is more concise to loop over references to containers instead of using explicit iteration methods
       --> serde_derive/src/internals/check.rs:202:20
        |
    202 |     for variant in variants.iter() {
        |                    ^^^^^^^^^^^^^^^ help: to write this more concisely, try: `variants`
        |
        = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#explicit_iter_loop
        = note: `-D clippy::explicit-iter-loop` implied by `-D clippy::pedantic`

    error: it is more concise to loop over references to containers instead of using explicit iteration methods
       --> serde_derive/src/bound.rs:262:28
        |
    262 |             for variant in variants.iter() {
        |                            ^^^^^^^^^^^^^^^ help: to write this more concisely, try: `variants`
        |
        = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#explicit_iter_loop

serde/src/private/de.rs

@@ -2758,7 +2758,7 @@ where
     where
         V: Visitor<'de>,
     {
-        for entry in self.0.iter_mut() {
+        for entry in self.0 {
             if let Some((key, value)) = flat_map_take_entry(entry, variants) {
                 return visitor.visit_enum(EnumDeserializer::new(key, Some(value)));
             }

serde_derive/src/bound.rs

@@ -259,7 +259,7 @@ pub fn with_bound(
     };
     match &cont.data {
         Data::Enum(variants) => {
-            for variant in variants.iter() {
+            for variant in variants {
                 let relevant_fields = variant
                     .fields
                     .iter()

serde_derive/src/internals/check.rs

@@ -195,7 +195,7 @@ fn check_variant_skip_attrs(cx: &Ctxt, cont: &Container) {
         Data::Struct(_, _) => return,
     };
 
-    for variant in variants.iter() {
+    for variant in variants {
         if variant.attrs.serialize_with().is_some() {
             if variant.attrs.skip_serializing() {
                 cx.error_spanned_by(