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The problem was that `lubA (cons <<< second (arr snd) <<<
checkConstructorAndLength "Zero" []) -< toSubterms t'` does many joins.
Each `⊔` joins the underlying fixpoint cache and with this the contained
grammar. These many joins on the same grammar caused the problems. I
changed the implementation for `⊔` for `Fixpoint`, such that it does not
join the fixpoint caches anymore, but instead threads them through.

 ```
Fixpoint f ⊔ Fixpoint g = Fixpoint $ \w -> proc ((i,o),x) -> do
  (o',t1) <- f w -< ((i,o),x)
  (o'',t2) <- g w -< ((i,o'),x)
  returnA -< (o'',case (t1,t2) of
    (Terminating y',Terminating v') -> Terminating (y' ⊔ v')
    (Terminating y',NonTerminating) -> Terminating y'
    (NonTerminating,Terminating v') -> Terminating v'
    (NonTerminating,NonTerminating) -> NonTerminating)
```

This is *not* the least upper bound, because the cache is first passed
to `f` and then to `g`, but is ok because `f` an `g` increase the
fixpoint cache monotonically.

A nice side effect is that we can drop the alphabet in the Const arrow,
because we now no longer need to top out way down in the interpreter
stack.

So the WildcardSemantics keep building.

This allows implementations to act on the (co)domain of the interp
arrows. For example, the GrammarSemantics can now actually call the
deduplication functions on the grammars.