Support precompiled NIFs

[scrogson]

This PR adds support for precompiled NIFs and updates a couple of issues
that have been pointed out by other issues and PRs.

I've added the necessary CI/release workflows for GitHub Actions as well, so all that's needed is to push a new tag, let it build, generate the checksum of the precompiled NIFs, and then publish to hex.

I'll add these steps to the README in a new commit later today.

For those using this service that want to take advantage of precompiled NIFs can use the following in the mean time:

  defp deps do
    [
      {:siwe, github: "royal-markets/siwe-ex", ref: "51c9c08"},
      # ...
    ]
  end