By Tobias Carroll Call it serendipity or synergy; regardless, it seems like this season of SNL has abounded with guests in addition to the guest host. These tend to be co-stars of the host for whatever their latest project is — Daniel Craig appeared in a few sketches when Rami Malek hosted, and last week Elizabeth Olsen joined her Dr. Strange costar Benedict Cumberbatch for a sketch. All of which is to say that Selena Gomez hosted SNL last night, and there’s a new season of Only Murders in the Building debuting next month. If you put money down on a Steve Martin appearance last night, well, yo…