Moving CHERIoT RTOS to a tickless model

The CHERIoT RTOS scheduler has transitioned to a tickless model, improving efficiency by calculating the next required scheduling decision and only requesting timer interrupts when necessary. The tickless approach led to unexpected slower performance in the test suite, which was resolved by refining the `thread_sleep` API to differentiate between yielding and sleeping. This resulted in a 30% faster test suite completion, with anticipated benefits for other workloads.

• CHERIoT RTOS moved to a tickless scheduler.
• Initial tickless implementation slowed tests.
• `thread_sleep` API was refined.
• Test suite became 30% faster.
• New scheduler helps save power.