perf(ext/headers): optimize headers iterable (#20155)

This PR makes more optimizations to headers iterable by removing
`ObjectEntries` which was consistently prominent in the flame graph when
benchmarking an express server.

**this PR**

```
cpu: 13th Gen Intel(R) Core(TM) i9-13900H
runtime: deno 1.36.1 (x86_64-unknown-linux-gnu)

benchmark         time (avg)        iter/s             (min … max)       p75       p99      p995
------------------------------------------------------------------ -----------------------------
headers iter        9.6 µs/iter     104,134.1   (8.74 µs … 131.31 µs)   9.47 µs  12.61 µs  17.81 µs
```

**main**
```
cpu: 13th Gen Intel(R) Core(TM) i9-13900H
runtime: deno 1.36.1 (x86_64-unknown-linux-gnu)

benchmark         time (avg)        iter/s             (min … max)       p75       p99      p995
------------------------------------------------------------------ -----------------------------
headers iter      12.87 µs/iter      77,675.9  (11.97 µs … 132.34 µs)  12.76 µs  16.49 µs   26.4 µs
```


```js
const headers = new Headers({
  "Content-Type": "application/json",
  "X-Content-Type": "application/json",
  "Date": "Thu, 14 Aug 2023 17:45:10 GMT",
  "X-Deno": "Deno",
  "Powered-By": "Deno",
  "Content-Encoding": "gzip",
  "Set-Cookie": "__Secure-ID=123; Secure; Domain=example.com",
  "Content-Length": "150",
  "Vary": "Accept-Encoding, Accept, X-Requested-With",
});

Deno.bench('headers iter', () => {
  [...headers]
})
```

1 files changed, 11 insertions, 14 deletions

diff --git a/ext/fetch/20_headers.js b/ext/fetch/20_headers.js
index 929eaf816..4e1729e01 100644
--- a/ext/fetch/20_headers.js
+++ b/ext/fetch/20_headers.js
@@ -26,7 +26,6 @@ const {
   ArrayPrototypeSort,
   ArrayPrototypeJoin,
   ArrayPrototypeSplice,
-  ObjectEntries,
   ObjectHasOwn,
   RegExpPrototypeTest,
   Symbol,
@@ -238,8 +237,8 @@ class Headers {
 
     // The order of steps are not similar to the ones suggested by the
     // spec but produce the same result.
-    const headers = {};
-    const cookies = [];
+    const seenHeaders = {};
+    const entries = [];
     for (let i = 0; i < list.length; ++i) {
       const entry = list[i];
       const name = byteLowerCase(entry[0]);
@@ -250,27 +249,25 @@ class Headers {
       // so must be given to the user as multiple headers.
       // The else block of the if statement is spec compliant again.
       if (name === "set-cookie") {
-        ArrayPrototypePush(cookies, [name, value]);
+        ArrayPrototypePush(entries, [name, value]);
       } else {
         // The following code has the same behaviour as getHeader()
         // at the end of loop. But it avoids looping through the entire
         // list to combine multiple values with same header name. It
         // instead gradually combines them as they are found.
-        let header = headers[name];
-        if (header && header.length > 0) {
-          header += "\x2C\x20" + value;
+        const seenHeaderIndex = seenHeaders[name];
+        if (seenHeaderIndex !== undefined) {
+          const entryValue = entries[seenHeaderIndex][1];
+          entries[seenHeaderIndex][1] = entryValue.length > 0
+            ? entryValue + "\x2C\x20" + value
+            : value;
         } else {
-          header = value;
+          seenHeaders[name] = entries.length; // store header index in entries array
+          ArrayPrototypePush(entries, [name, value]);
         }
-        headers[name] = header;
       }
     }
 
-    const entries = ObjectEntries(headers);
-    for (let i = 0; i < cookies.length; ++i) {
-      ArrayPrototypePush(entries, cookies[i]);
-    }
-
     ArrayPrototypeSort(
       entries,
       (a, b) => {