Use log' instead of putStrLn

[dhartunian]

These logs end up running over regular log statements and make it harder
to grep specific lines.

[emhoracek]

Looks like we need to wrap the tests in withStdoutLogging. I think that's why we didn't use log' to begin with. I have a branch in progress to add an initLarceny that would be a good place to allow this sort of stuff to be configurable -- idea is in #44 .