systick: Refactoring.

This change rewrites systick module a bit to use new ioreg macro, changes  `systick::setup` a bit to eliminate "boolean trap", exposes tenms() to the user, so the user can read and manipulate it.
Depends. For standalone applications (not using any rust-provided scheduler), it might be useful to check if there's any calibrated value. Is there any cost having it public?