Enhance `nthnext` to understand HSequential inputs

Signed-off-by: Ambrose Bonnaire-Sergeant <abonnairesergeant@gmail.com>
`nthnext` now properly refines a union of HSequentials

Signed-off-by: Ambrose Bonnaire-Sergeant <abonnairesergeant@gmail.com>
`nthnext` of nil always returns nil

Signed-off-by: Ambrose Bonnaire-Sergeant <abonnairesergeant@gmail.com>