Initial commit of Haskell constructible library

Signed-off-by: Anders Kaseorg <andersk@mit.edu>
Add complex-generic integration

Signed-off-by: Anders Kaseorg <andersk@mit.edu>
Move extensions to {-# LANGUAGE #-} pragma

Hackage doesn’t understand extensions: DataKinds yet.

Signed-off-by: Anders Kaseorg <andersk@mit.edu>