Round Robin Scheduler for Tock
This scheduler is specifically a Round Robin Scheduler with Interrupts.
See: https://www.eecs.umich.edu/courses/eecs461/lecture/SWArchitecture.pdf for details.
When hardware interrupts occur while a userspace process is executing, this scheduler executes the top half of the interrupt, and then stops executing the userspace process immediately and handles the bottom half of the interrupt. This design decision was made to mimic the behavior of the original Tock scheduler. In order to ensure fair use of timeslices, when userspace processes are interrupted the scheduler timer is paused, and the same process is resumed with the same scheduler timer value from when it was interrupted.