using one cothread for each executable device. This functionality is
strictly internal and should result in no changes in scheduling
behavior. However, it now becomes possible to exit out of a device's
execution in the midst of an instruction by calling
machine().scheduler().make_active(). Note that this is somewhat dangerous
for cores that have logic at the start of their execute loop to check for
interrupts or other conditions, as those checks will be skipped upon
resume, so consider this sort of early exiting to be highly experimental
for now. [Aaron Giles, Miodrag Milanovic, byuu]