Implement itimer syscalls with generic timer handling framework

[arihant2math]

Replaces #267 with a generic callback framework to handle arbitrary timers.

Interval timers are an POSIX feature and come in three forms (real, virtual, prof). This PR only implements itimers on the wall clock.

I've moved all the clock syscalls to src/clock/syscalls/

Known issues/followups/etc.:

• We don't move tasks between CPUs, but if we did, removing an itimer might fail because removing from non-local wakeup queues isn't supported.
• No support for virtual/prof, they likely can be implemented by doing a scheduler check before starting a task, however that requires some infrastructure.
• Other timers depend on the current date, which can be adjusted and is different from wall time, so we need a separate wakeup queue for that.
• Linux helps prevent interrupt spam by preventing interrupts from being scheduling too early in the future (i.e. interrupts must be scheduled atleast X nanoseconds in the future) and by grouping interrupts that are within ~50ns of each other, we probably should do this too.

Testing: usertest

[hexagonal-sun]

Should we also remove scheduled timers on task exit? in exit.rs, just iterate the namespace and call remove_scheduled_timer() for any that are set.

[hexagonal-sun]

Is this correct? According to the man page for timer_create():

Timers are not inherited by the child of a [fork(2)](https://man7.org/linux/man-pages/man2/fork.2.html), and are
       disarmed and deleted during an [execve(2)](https://man7.org/linux/man-pages/man2/execve.2.html).