add widening operator

[andreinaku]

def anytest():
    c = [3]
    while rand_cond:
        c = [c]
    return c

cases:

• too many nestings: list < list < list < list < ... > > > > -> list < top >
• too many sum types: int + float + str + ... -> top

but how? need be careful in the worklist algorithm, so I don't get to list < list < top > > and start over again. (check that condition is correct for modified)