CLN: Change assert_(a [not] in b) to specialized forms
Work on #6175. Changes instances of assert_(a [not] in b) to specialized assert[Not]In(a, b).

Busy week; does this need to be rebased?

Is there any easy way on this screen to tell if a PR has conflicts? When creating a PR there is a prominent icon, but I am not seeing anything obvious now that the PR has been created.
 If on a PR is say 'this PR can be automatically merged' then no conflicts
