alternative printing for the REPL

instead of using print(), use str().  Also, be even more cautious
about cleaning up in printToString; previously, errors in printing
would lead to a sink to a closed fifo...

Printing is still not really right, and I forsee that we will end up
writing our own printer to get something more lispy.  But at least
this approach tends not to dump huge tables into the REPL.