Minor cleanup
- Simplify
mkBuildExpr, the functionnewTyVarswas called only on a one-element list. - TTG: use
noExtConin more places. This is more future-proof. - In
zonkExpr, panic instead of printing a warning.
mkBuildExpr, the function newTyVars was called only on a one-element list.noExtCon in more places. This is more future-proof.zonkExpr, panic instead of printing a warning.