Since we require meson 1.3(.2), we can modernize the usage a bit,
as well as improve some logic
- use / instead of join_paths
- use option.allowed() instead of !option.disable()
- use cxx.get_supported_arguments
- use fs.copyfile instead of configure_file
- show a summary at the end of the configuration
Implement accessing the MIME-parts + docs + test.
Implement saving attachments to file.
Implement creating messages from files.
Refactor / rename functions to be more uniform.
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.
