[racket] Programmatically start a big-bang?

> Here is a sketch: [...]

Great, thanks!  my-bb is enough to get some simple cases working
already, and it should work for everything I have in mind.

I didn't realize that I could use (require 2tdhp/private/world) like
that; world% is what I thought I needed and I thought `private` meant
I couldn't get to it so I didn't try that hard.  Should I be worried
about things under private/ collection subdirectories having
less-than-stable interfaces?

Thanks for suggestions, all.