Initial version of argon2id gem

Inspired by the recommendation for the Argon2id password-hashing
function in [The Copenhagen Book][] and [OWASP Password Storage Cheat
Sheet][], this aims to provide native, pre-compiled bindings to the
[reference C implementation of Argon2][Argon2].

There is an existing [Ruby Argon2 gem][] but that provides bindings via
FFI and I was keen to re-use my experience developing the [re2][] gem.

  [The Copenhagen Book]: https://thecopenhagenbook.com/password-authentication
  [OWASP Password Storage Cheat Sheet]: https://cheatsheetseries.owasp.org/cheatsheets/Password_Storage_Cheat_Sheet.html#argon2id
  [Argon2]: https://github.com/P-H-C/phc-winner-argon2
  [Ruby Argon2 gem]: https://github.com/technion/ruby-argon2)
  [re2]: https://github.com/mudge/re2

.github/workflows/precompile-gem.yml

@@ -0,0 +1,23 @@
+name: Precompile native gem
+
+on:
+  workflow_call:
+    inputs:
+      platform:
+        required: true
+        type: string
+
+jobs:
+  precompile-gem:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: ruby/setup-ruby@v1
+        with:
+          ruby-version: "3.3"
+          bundler-cache: true
+      - run: bundle exec rake gem:${{ inputs.platform }}
+      - uses: actions/upload-artifact@v4
+        with:
+          name: "cruby-${{ inputs.platform }}-gem"
+          path: pkg/*.gem