simplify_logic now converts to cnf by default

Currently, Implies(0,0) returns Implies(0,0) instead of True.
Classes like And, Not and Or convert integers to True/False, but classes like Implies and Equivalent do not. simplify_logic depends on 0, 1 being treated as False, True on substituion
Hence, simplify_logic has been modified to convert instances of these classes to cnf/dnf form to avoid confusion.