Implement message->alist; i.e. to convert the mu4e-style plist into an idiomatic
alist. Add it as a message slot, initializing it lazily.
Update the message accessors to use the alist.
Add tests, docs.
Implement accessing the MIME-parts + docs + test.
Implement saving attachments to file.
Implement creating messages from files.
Refactor / rename functions to be more uniform.
Replace the iso-date->time-t and v.v. functions with something more
customizable. Add more tests.
Use some (internal for now) %preferences variable for the defaults. TBD... maybe
should become a fluid?
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.
