[dataimport] test for a value is in a set and insertion in a set should live together also inline some methods on the way, in the hope to make the public api clearer at some point, and only put valuable values in those sets so we don't have to endlessly test for consistency.

../../doc/book usr/share/doc/cubicweb-documentation
../../doc/_build/html usr/share/doc/cubicweb-documentation