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.
Or

```
match reg::SYSTICK.calib.tenms() {
  0 => None,
  val => Some(val)
}
```