Optimize replace_headers to avoid copies of keys

Sometimes, I'm amazed at what non lexical lifetimes allows.

src/util.rs

@@ -1,4 +1,4 @@
-use crate::header::HeaderMap;
+use crate::header::{Entry, HeaderMap, OccupiedEntry};
 
 // xor-shift
 #[cfg(not(target_arch = "wasm32"))]
@@ -42,21 +42,23 @@ pub(crate) fn replace_headers(dst: &mut HeaderMap, src: HeaderMap) {
     // The first time a name is yielded, it will be Some(name), and if
     // there are more values with the same name, the next yield will be
     // None.
-    //
-    // TODO: a complex exercise would be to optimize this to only
-    // require 1 hash/lookup of the key, but doing something fancy
-    // with header::Entry...
 
-    let mut prev_name = None;
+    let mut prev_entry: Option<OccupiedEntry<_>> = None;
     for (key, value) in src {
         match key {
-            Some(key) => {
-                dst.insert(key.clone(), value);
-                prev_name = Some(key);
+            Some(key) => match dst.entry(key) {
+                Entry::Occupied(mut e) => {
+                    e.insert(value);
+                    prev_entry = Some(e);
                 }
-            None => match prev_name {
-                Some(ref key) => {
-                    dst.append(key.clone(), value);
+                Entry::Vacant(e) => {
+                    let e = e.insert_entry(value);
+                    prev_entry = Some(e);
+                }
+            },
+            None => match prev_entry {
+                Some(ref mut entry) => {
+                    entry.append(value);
                 }
                 None => unreachable!("HeaderMap::into_iter yielded None first"),
             },