fix(js): use pure JS SHA256 library, refactor (#471)

* fix(js): use pure JS SHA256 library, refactor

Closes #458

Additionally, I made a horrifying discovery: Firefox seems to actively
hinder performance if you are using more than one Worker per page. It
does not spread the load out across cores like I expected. Instead it
seems to make that one Worker thrash and have to constantly context
switch, which caused a lot of slowdown.

The benchmarks in #155 continue to be the best contribution ever made to
Anubis. What clued me into there being a problem here was the fact that
the "slow" algorithm was faster than the "fast" algorithm on my laptop.
This made no intuitive sense to me so I dug further.

Either way I think this is a Firefox bug at its core, but for now we
have to work around it by doing the hacky terrible thing that I hate.

I also swapped the SHA256 operations to @aws-crypto/sha256-js on the
advice of a trusted cryptography expert. I don't know what performance
differences this makes, but I'm getting 150-225 kilohashes per second,
which is pretty dang good.

Signed-off-by: Xe Iaso <me@xeiaso.net>

* fix(js): apply suggestions from code review

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>
Signed-off-by: Xe Iaso <me@xeiaso.net>

* fix(js): use fast algo for fast worker

Signed-off-by: Xe Iaso <me@xeiaso.net>

---------

Signed-off-by: Xe Iaso <me@xeiaso.net>
Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

web/js/worker/slow.mjs

@@ -0,0 +1,35 @@
+import { Sha256 } from '@aws-crypto/sha256-js';
+
+const sha256 = (text) => {
+  const hash = new Sha256();
+  hash.update(text);
+  return hash.digest()
+    .then((result) =>
+      Array.from(new Uint8Array(result))
+        .map((c) => c.toString(16).padStart(2, "0"))
+        .join(""),
+    );
+};
+
+addEventListener('message', async (event) => {
+  let data = event.data.data;
+  let difficulty = event.data.difficulty;
+
+  let hash;
+  let nonce = 0;
+  do {
+    if ((nonce & 1023) === 0) {
+      postMessage(nonce);
+    }
+    hash = await sha256(data + nonce++);
+  } while (hash.substring(0, difficulty) !== Array(difficulty + 1).join('0'));
+
+  nonce -= 1; // last nonce was post-incremented
+
+  postMessage({
+    hash,
+    data,
+    difficulty,
+    nonce,
+  });
+});