Stop using integer sets to represent groups of share sizes for pass calculations

_zkapauthorizer.storage_common.required_passes takes share_sizes as an argument. This is a set of int where each element is meant to represent the size of one share. The number of passes required is then the total of the sizes divided by the value of a pass.

The problem, of course, is that shares for a storage index are mostly (if not always) the same size so a "set" like {n, n, n} is actually just {n} which looks like it is 1/3rd the price.