Allow quibs in overriding assignments

[rkishony]

if Q is a quib, allow:
Q[ind] = val
where both ind and val can be quibs

today we are transforming to values.
but we can instead keep the assignment 'functional'

this can be very helpful in definitions like:

n = iquib(10)
z = np.arange(n)
z(z % 3) = 0