#8 support weighted index distributions

We should support the [`WeightedAliasIndex`](https://docs.rs/rand_distr/0.4.3/rand_distr/weighted_alias/struct.WeightedAliasIndex.html) distribution. While it's possible the memory usage is sufficiently high and the empirical distributions sufficiently simple that we opt to use a parameterized distribution instead, using the empirical distributions directly will likely be easier to get working, and could be good enough or necessary. We opt for `WeightedAliasIndex` instead of [`WeightedIndex`](https://rust-random.github.io/rand/rand/distributions/struct.WeightedIndex.html) because it supports `O(1)` sampling instead of `O(log n)` at the cost of higher initialization, exactly the kind of tradeoff that makes sense in a Shadow experiment.

Resolved via ec9d2eb921 and edf8a376f2