Avoids directly relying on the global system instance and instead makes
an arbitrary system instance an explicit dependency on construction.

This also allows removing dependencies on some global accessor functions
as well.

This retrieves:

if (curr_thread == handle_thread) {
   result = total_thread_ticks + (hardware_tick_count - last_context_switch_ticks);
} else if (curr_thread == handle_thread && sub_id == current_core_index) {
   result = hardware_tick_count - last_context_switch_ticks;
}

It doesn't make sense to allow a scheduler to be constructed around a
null pointer.

Gets all of these types and interfaces out of the global namespace.

This function doesn't modify instance state, so the const qualifier can
be added to it.

General moving to keep kernel object types separate from the direct
kernel code. Also essentially a preliminary cleanup before eliminating
global kernel state in the kernel code.

Removes unnecessary direct dependencies in some headers and also gets
rid of indirect dependencies that were being relied on to be included.