Don't use new assumptions in the sets module
Implement a symbolic object for the relation "is an element of"

1. fixes #6194
2. contains methods of set classes were modified to return
   BooleanAtom's, not True/False
Printing support for Contains class
Fix some asserts to check if foo.contains(bar) is S.true
Merge pull request #7989 from skirpichev/contains

Implement a symbolic object for the relation "is an element of"