rust: hrtimer: Add TimerCallbackContext and ::forward()

With Linux's hrtimer API, certain functions require we either acquire
proper locking to call specific methods - or that we call said methods from
the context of the timer callback. hrtimer_forward() is one of these
functions, so we start by adding a new TimerCallbackContext type which
provides a way of calling these methods that is inaccessible outside of
hrtimer callbacks.

Signed-off-by: Lyude Paul <lyude@redhat.com>