add theft_random_shuffle to API

[silentbicycle]

Along with theft_random_choice and theft_random_bits, there should be a function that takes a length and a pointer to a size_t array at least length cells long, and writes shuffled indices into that array.

void theft_random_shuffle(struct theft *t, size_t length, size_t *indices);

This could be done with Fisher-Yates.