algebra.py: fix a few simple bugs
algebra.py: fix a few simple bugs

`preorder` was calling `postorder` on the children

`Join` didn't override `__repr__` and so wouldn't print

Signed-off-by: Daniel Halperin <1a02732a15f637c31107256cbee5819e36aa7c67@cs.washington.edu>

uwescience/raco,uwescience/raco,uwescience/raco,uwescience/raco,uwescience/raco