Ephemerons, self-referentality in weak hashtables

Hello, I'm finally taking some time to port Goblins to Guile, in-between other tasks anyway. In Goblins there is a weak hashtable that maps current actor references to their current behavior. I found that for self-referential actors, I needed ephemerons for GC stuff to work right. In this old t

Re: Add ‘expt’ to the list of effect-free primitives

Applied to main, thanks.