Initial priority-based scheduler

- Scheduler now has multiple linked_lists of processes at different
  priorities
- Process structure improvements
- scheduler::tick() and scheduler::schedule() separation

src/drivers/nulldrv/main.s

@@ -3,7 +3,8 @@ _start:
 	xor rbp, rbp	; Sentinel rbp
 
 .loop:
-	mov rax, 0		; DEBUG syscall
+	;mov rax, 1		; DEBUG syscall
+	mov rax, 0		; NOOP syscall
 	syscall
 
 	jmp .loop