Models have `incr_<SEM>` and `<SEM>_set?` to interact with semaphores.
Progs also have `incr_<SEM>`, `decr_<SEM>`, and `when_<SEM>_set?` to
interact with semaphores. The `when_<SEM>_set?` method accepts a block
and executes it when the semaphore is set, so we can't negate the
condition. Additionally, it doesn't offer much advantage over
`<SEM>_set?`. Having two different interfaces makes things more complex,
so we might consider removing `when_<SEM>_set?`. As a first step, I
added a `<SEM>_set?` helper to the base prog. I used it to check for the
absence of the destroying semaphore instead of checking via its subject.
Since we have the `incr` method on both the prog and model, we can also
have the `set?` method on both.
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