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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.

This alleviates the need to define a `Complete [Term]` instance, which
does not make sense. The requirement for this instance comes from
`eval'`, which requires `Arrowtry (t,[t]) (t,[t]) (t,[t]) c` due to
`some` and `all`.

We circumvent this now by using a new `Completion` arrow transformer,
whose `Complete` instance requires a `Complete (FreeComplete y)`
instance such that there is no requirement for a `Complete y` instance.

With this, we can derive a `Complete` instance for the `Interp a b`
arrow that does not require `Complete b`, but rather `PreOrd b`.

It was found to be unsound (see
a1f5b024), and can be replaced by this
implementation of `<+>`.