Add TODO in cabal file for adding version constraints to dependencies.
2 build fixes.

 * Add newly missing import of Control.Exception in Cfipu.hs.
 * Replace mtl's pure state monad constructor with 'state', previously
   @State@.
Fix formatting of Cfipu's export list, and export main, fixing build error.
Forgot to update cabal-version constraint to 1.18.  Fixed.