Subtype for State to improve compatiblity with Crux

[kevinbradner]

Please see the commit below for suggested change and an explanation. My fork of this repository does not have a clean copy of the original, so I don't know of any convenient way to submit a PR myself. It's a very small change, so it should be easy to make regardless.

https://github.com/JuliaAcademy/Decision-Making-Under-Uncertainty/commit/3acaec2585c3d5485a1b1e72b427ef51e648e574

Thanks for making these tutorials! I used them to learn how to translate some research code from Python to Julia.