Fix infinite vp

[pscott]

Adds a assert_no_duplicates function that will throw if duplicates are found in an array. This function is doing this the "dumb way" (i.e. for each element, loop through the array and make sure there a no duplicates, resulting in O(n^2) complexity). The good thing about this is that no builtins are used whatsoever: so it's step intensive but builtin lightweight.
Another way we could do this is by using the bitwise builtin. Since most spaces will probably not support a gazilion voting strategies, I think we're fine with that, but we could indeed other ways of doing this in the future (sorting by increasing order for example to make lookup faster...)

Call this function in get_cumulative_voting_power to make sure voting strategies are unique.

Closes #298

[Orland0x]

LGTM!