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.
Sadly I don't think `//=` will even work here. We [made up](https://github.com/hackndev/zinc/pull/135) that syntax specifically for `ioregs!`. It might be nice to propose it though.