sum-of-multiples: handle cardinailty > 3 correctly

[yawpitch]

It is possible to approach this exercises using an entirely arithmetic approach instead of a container type. In this approach the total of the arithmetic series of each factor must be compensated for overlaps using the inclusion-exclusion principle; the current body of tests handle only up to a cardinality of 3, but it is quite easy to get something passing a cardinality of 3 without realizing it will fail with cardinality of 4 or more. A test with 5 prime factors will demonstrate the ability of the solution to handle the problem generally.