trailers/Deno.trailers support for Deno.serve and fetch

[mmastrac]

The WhatWG discussion on trailers in requests and responses is picking up a bit and it looks like CloudFlare is interested in implementing this as well.

We should implement trailers support for requests and responses behind a Deno-specific symbol that we can then swap out for the full API when standardized:

• We add an --unstable only Deno.trailers unique symbol.
• We add support for this symbol to Response, Request and the associated *Init objects. The field value would be Promise<Headers> or Promise<HeadersInit>, respectively
• If we get support for this in the standard, Deno.trailers can be turned from a symbol into a string with the value trailers and marked as deprecated.
• If a promise was provided in RequestInit for fetchor ResponseInit for Deno.serve, we'd await that promise before finalizing the associated HTTP transaction.

Initially this would only be supported for HTTP/2, however we could attempt to push support upstream to hyper for HTTP/1. I don't think that will be necessary, however.

This would allow for something like this to work:

Deno.serve(req => {
  const bodyHash = sha256(req.body);
  const hash = (await req[Deno.trailers])["SHA1-Of-Body"];
  validateBody(hash, bodyHash);
  const { stream, streamHash } = streamResponse(req.url);
  return new Response({ body: stream, [Deno.trailers]: (async () => ({ "SHA1-Of-Body": await streamHash }))() });
});