Lazy cartesian products

[01mf02]

This PR makes the evaluation of cartesian products lazy in both arguments.

Previously, only the left-hand side of binary filters (such as addition) were evaluated lazily. As a consequence,

limit(3; recurse(.+1) + 0)

would terminate, but

limit(3; 0 + recurse(.+1))

would not. With this PR, both variants terminate.

As a downside, the results of the right-hand side are not cached anymore.
That is, (0, 1) + expensive_f will now evaluate expensive_f twice, whereas it was only evaluated once before.
However, I suppose that the impact of this is minimal, because few people would use multiple inputs in conjunction with "expensive" functions as arguments to binary operators.