Representing random sequences

Say you call `rand` on a stochastic sequence. It might be infinite, so you can't just call `rand` all the way down. But if you leave it as a thunk, you lose reproducibility.

The best way I can think of to get around this gracefully is to have a new struct to hold the instantiation. Something like

```julia
struct SequenceSample{R, S}
rng::R
seq::S
end
```

Also, iterator types get a little funny. One possibility is to mostly organize around `Base.Generator`. But there are lots of details for all of this, and I think it could get tricky.