function to show the effective output datatype after an extension

something like

```
extensible [d| data A = A |]

{-
other stuff, plus:
printExtendA :: String -- ^ datatype name
             -> [Name] -- ^ extra tyvars
             -> ExtA -> Dec
-}
```

```
>>> ppr $ printExtendA "A" [] $ defaultExtA {typeA = Just [[t|Int|]]}
data A = A Int
```