Write an ML function cross which, given two lists, computes the list of their cross-products.

Namely, given the inputs [1,2,3] and [4,5,6], it computes [(1,4), (1,5), (1,6), (2,4), (2,5), (2,6), (3,4), (3,5), (3,6)].

fun cross a b = ...