CLN: Change assert_(a in b) and 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).
Merge pull request #6381 from bwignall/assert_6175_newAsserts_2

CLN: Change assert_(a in b) and assert_(a not in b) to specialized forms