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.
