Implement support for "header" and "body" procedures, with require loading the
message file from disk, and create a foreign object for the message.
We keep those alive in a vector, and hook up a finalizer.
Update docs & tests as well.
This implements the new scm/guile bindings for mu, to replace the deprecated
guile/ (at some point in the future).
For now, we allow for creating a guile shell with mu support.
