plugins/muc/muc.lib: Move (de)construct_stanza_id into `handle_iq_to_occupant`
plugins/muc/muc.lib: Move (de)construct_stanza_id into `handle_iq_to_occupant`

It is the only place they were used; and I left the old function names in as comments.
One reason for doing this was to reduce accesses to _occupants; which may be in a database in future revisions

sarumjanuch/prosody,sarumjanuch/prosody