Tavor needs a rand.Uint64n method or the move to bigint

[zimmski]

The amount of permutations can get big very fast. Currently we use uint64 where we can but there is no rand.Uint64n only a rand.Int63n which often fails because of the need of a int64 conversation which converts very high numbers to negative numbers failing the call to every rand method because they want positive numbers.

Another solution would be to use bigints which would make everything easier since we can then use arbitrary large numbers.