Separate package for extraction modules

[liyishuai]

Description of the problem

As mentioned in QuickChick/QuickChick#294, QuickChick extracts Z to either integer or big integer based on the underlying Coq version, because the ideal extraction module only ships with Coq >= 8.15.

Consider splitting extraction modules into a separate library so that older versions of Coq can use the new extractions?

Furthermore: Should we version the entire standard library asynchronously from the core?

Coq Version

Version-generic.

[anton-trunov]

This would be great to have! It's kind of a blocker for a project I'm working on to support Coq 8.15.