raco: fix string utility functions with containers

We want to stringify operators in a way that recursively stringifies
containers. I had previously hacked this up to work for non-nested
lists, but of course, NaryJoin uses nested lists. Fix this by rewriting
real_str to work with arbitrary containers.
utility.real_str: test, and requisite bug fixes
test_utility: remove erroneous comment
test_utility: remove duplicate test
Merge pull request #304 from uwescience/redo-stringify

raco: fix string utility functions with containers