perf(ext/headers): optimize headers iterable

[marcosc90]

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

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]
})

[mmastrac]

LGTM.