perf(ext/http): Reduce size of ResponseBytesInner

[nathanwhit]

I noticed set_response_body was unexpectedly hot in profiles, with most of the time being spent in memmove.

It turns out that ResponseBytesInner was massive (5624 bytes), so every time we moved a ResponseBytesInner (for instance in set_response_body) we were doing a >5kb memmove, which adds up pretty quickly.

This PR boxes the two larger variants (the compression streams), shrinking ResponseBytesInner to a reasonable 48 bytes.

---

Benchmarked with a simple hello world server:

// hello-server.ts
Deno.serve((_req) => {
  return new Response("Hello world");
});
// run with `deno run -A hello-server.ts`
// in separate terminal `wrk -d 10s http://127.0.0.1:8000`

Main:

Running 10s test @ http://127.0.0.1:8000/
  2 threads and 10 connections
  Thread Stats   Avg      Stdev     Max   +/- Stdev
    Latency    53.39us    9.53us   0.98ms   92.78%
    Req/Sec    86.57k     3.56k   91.58k    91.09%
  1739319 requests in 10.10s, 248.81MB read
Requests/sec: 172220.92
Transfer/sec:     24.64MB

This PR:

Running 10s test @ http://127.0.0.1:8000/
  2 threads and 10 connections
  Thread Stats   Avg      Stdev     Max   +/- Stdev
    Latency    45.44us    8.49us   0.91ms   90.04%
    Req/Sec   100.65k     2.26k  102.65k    96.53%
  2022296 requests in 10.10s, 289.29MB read
Requests/sec: 200226.20
Transfer/sec:     28.64MB

So a nice ~15% bump. (With response body compression, the gain is ~10% for gzip and neutral for brotli)

[bartlomieju]

I see 8-9% speed up on M1 Max:
main

wrk -d 10s --latency http://localhost:8000
Running 10s test @ http://localhost:8000
  2 threads and 10 connections
  Thread Stats   Avg      Stdev     Max   +/- Stdev
    Latency    75.96us   68.31us   3.40ms   99.36%
    Req/Sec    60.72k     2.53k   62.38k    97.03%
  Latency Distribution
     50%   73.00us
     75%   84.00us
     90%   97.00us
     99%  131.00us
  1220434 requests in 10.10s, 174.58MB read
Requests/sec: 120836.25
Transfer/sec:     17.29MB

This PR:

wrk -d 10s --latency http://localhost:8000
Running 10s test @ http://localhost:8000
  2 threads and 10 connections
  Thread Stats   Avg      Stdev     Max   +/- Stdev
    Latency    67.71us   34.28us   3.12ms   92.86%
    Req/Sec    65.45k     1.55k   67.33k    93.07%
  Latency Distribution
     50%   68.00us
     75%   77.00us
     90%   89.00us
     99%  115.00us
  1315388 requests in 10.10s, 188.17MB read
Requests/sec: 130242.23
Transfer/sec:     18.63MB

[bartlomieju]

LGTM