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.
Right, kill the doc comment. I like that you killed the parameter; it makes things nicely more explicit to first configure the peripheral with `setup` and then `enable_irq`.