Might it make sense to require main to be Eff _ _ in the --main module (if one is specified) when running psc-bundle? This has tripped me up a couple of times and can be quite hard to diagnose. I accidentally had main as an Aff instead of an Eff, which was extra confusing because my program appeared to run normally and then threw an error right at the end.

Also, if we are going to do this, should we move the Eff type into Prim?

See also #1350.