thorium_actor: send the output of thorium_actor_log in stderr

Reported-by: Fangfang Xia <fangfang@gmail.com>
Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>
thorium: add thorium_nop that does nothing at all

Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>
unitig_visitor: use %s when printing a string

Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>
thorium: thorium_actor_log is now a macro that can be disabled

Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>