Global key-value metadata storage for Scryer Prolog

It's been a long time since I've programmed Prolog so I'm not sure if these ideas are even relevant (global state might be more than enough!) ; but at first impressions from the readme this really reminds me of ETS

https://www.erlang.org/doc/apps/stdlib/ets.html

Essentially system-global state under named tables.

Have/did you consider having named `env` instances? This would allow for say, feature flags to be kept in a `features` set of relations.

You could also draw from Erlang's 'match specs' and query with a predicate over the keys.