message:recipient() may now return an array style table
holding the recipient list, if there is more than a single
recipient on the message.
Since this can be somewhat ambiguous/frustrating to work with,
there is now also a message:recipient_list() that will always
return an array style table, even if it holds just a single
element.
The included helpers have been updated to use `message:recipient_list`.
message:set_recipient() will now optionally accept an array
style table holding the recipient list to be set.
