Use ztimer_ondemand in the wrappers

[chrysn]

The ztimer abstractions were already built assuming it, but now that we have acquire and release, this can probably be improved.

(This is mainly a note-to-self because it's sailing past me in the master branch changes list right now, and unlike often I can't just do it now).

[chrysn]

When fixing this, also consider the new ztimer_stopwatch. (Not sure if that's better used throught the new convenience functions, or whether we'll just have something equivalent in Rust).

[chrysn]

An equivalent to ztimer_stopwatch is being added in #103 (but not using ztimer_stopwatch for lack of overflow detection).

We can't really add ztimer_ondemand as that is really a negative implementation, which is a wholly different issue (RIOT-OS/RIOT#20824).