[Feature request] Gate extractions to F* and ProVerif behind a single feature `hax`

[jschneider-bensch]

#102 and #107 both introduce features to gate extraction to ProVerif and F*, respectively. Both features are justified by an optional dependency on hax-lib-macros and nothing else. This suggests unifying the features into a single hax feature that guards any hax extraction specific code and dependencies.

This should also clean up redundancies in the hax driver.

[franziskuskiefer]

hax itself sets the hax config that could be re-used for anything hax related.