More X.509 bindings

This patch set makes it possible to generate X.509 private keys and certificates with Guile. I haven't written much C code or used Guile's C API in quite awhile but I tried my best. The license headers indicate that there's FSF copyright assignment involved here, which is fine because I filed my paperwork for it years ago. I also threw in a guix.scm file to make it easy for Guix users to create a dev environment using guix shell, which has become the norm for Guile projects.

The Alpine tarball CI job appears to be failing due to a reason unrelated to my changes.