Fix code duplication: `relnext_quantify_policy`

[SSoelvsten]

With one of the optimisations in #502 , we end up adding some source-code copy by creating the relnext_quantify_policy which is a copy-paste of the regular BDD quantify policy ( #675 ). In the long-run this ought to be fixed. To do so, I'd suggest one merges all source files in <adiar/bdd/...> into a single file. This may also improve running time by inlining more functions.