perf(ext/headers): cache iterableHeaders for immutable Headers

[marcosc90]

This PR caches _iterableHeaders for immutable Headers increasing the performance of fetch & server if headers are iterated.

Should close #19466

I only cached immutable headers to address this comment #19466 (comment) since I didn't find any occurrence of header mutation on immutable headers. We can discuss caching for non-immutable, but I think this is a great first step.

BENCHMARK

Server

const addr = Deno.args[0] ?? "127.0.0.1:4500";
const [hostname, port] = addr.split(":");
const { serve } = Deno;

serve({ hostname, port: Number(port), reusePort: true }, (req) => {
  const headers = [...req.headers]; // req.headers are immutable, cannot set/append/delete
  return new Response("ok");
});

Used wrk with 5 headers

wrk -d 10s --latency -H "X-Deno: true" -H "Accept: application/json" -H "X-Foo: bar" -H "User-Agent: wrk" -H "Accept-Encoding: gzip, br" http:https://127.0.0.1:4500

This patch

Running 10s test @ http:https://127.0.0.1:4500
  2 threads and 10 connections
  Thread Stats   Avg      Stdev     Max   +/- Stdev
    Latency    70.18us   22.89us 679.00us   81.37%
    Req/Sec    71.55k     9.69k   82.18k    89.60%
  Latency Distribution
     50%   59.00us
     75%   89.00us
     90%   98.00us
     99%  159.00us
  1437891 requests in 10.10s, 193.35MB read
Requests/sec: 142369.83
Transfer/sec:     19.14MB

main

Running 10s test @ http:https://127.0.0.1:4500
  2 threads and 10 connections
  Thread Stats   Avg      Stdev     Max   +/- Stdev
    Latency   112.78us   36.47us   2.09ms   77.99%
    Req/Sec    44.30k     1.65k   49.14k    74.26%
  Latency Distribution
     50%   99.00us
     75%  136.00us
     90%  162.00us
     99%  213.00us
  890588 requests in 10.10s, 118.91MB read
Requests/sec:  88176.37
Transfer/sec:     11.77MB

fetch

const res = await fetch('http:https://127.0.0.1:4500');
Deno.bench("Headers iterator", () => {
  const i = [...res.headers]; // res.headers are immutable, cannot set/append/delete
});

this patch

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 iterator      329.5 ns/iter   3,034,909.0 (318.55 ns … 364.34 ns)  331.1 ns 355.72 ns 364.34 ns

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 iterator       2.59 µs/iter     386,372.1     (2.56 µs … 2.68 µs)   2.59 µs   2.68 µs   2.68 µs

[mmastrac]

LGTM