* Switch to crossbeam-channel + rayon::spawn
* Remove thread_spawn for evaluation altogether
This allows to avoid a deadlock when there is only one Rayon thread, and doesn't sacrifice performance, since the caller of .get_result() had to always block on the iterator to be finished anyway, and all the messages are already sent from separate threads.
* Fix `verbose_mode` test
This one is easier to "fix", since we're in control of the rayon pool - just adding one extra thread to the default number to make sure we always can one root "spawn" call without stealing from the actual pool.
e38c759563