[racket-dev] , en and enter! sometimes do nothing, and it's changing over releases?

Sorry that it has taken me so long to join in and that I overlooked the
PR way back in September.

I think the problem is that `enter-load/use-compiled' doesn't follow
the protocol for a load handler, which changed slightly for submodules.
Specifically, if the second argument to the load handler is a list that
starts with #f, the load handler isn't supposed to try to load code
from source.

I've pushed a repair that works for the example in PR 13096 and for
`(enter! slideshow/pict)'.