Check that the body of while-loops contains at least one `step`

[ngernest]

As discussed today (11/11), we want to disallow loops that don't contain at least one step() in its body, otherwise the loop might not converge within a single step.

[ekiwi]

This might be hard to do statically, though. So not a huge priority at the moment.

[ngernest]

Ah yes, good point! Yeah, if we were to implement this, this would most likely be a dynamic check.

[ekiwi]

I think in the new state machine IR that I want to discuss next meeting, this check might actually fall out of one of the normalization steps. I.e., when we try to get rid of all state transitions that do not consume a step, that process will fail if there is a loop without a step in its body (or with a way to execute the body without a step).