Export Fay type and thunk forcing of polymorphic types used with the FFI

For the `Num` polymorphic things like

    max :: (Num a) => a -> a -> a
    max = ffi "Math.max(%1,%2)"

The arguments won't be forced. So it's likely to get a `null` or something return value.

Also exported the `Fay` type from `Prelude`.