Expose Animate.Msg fuzzer.

[akoppela]

We are using this library for animations. It's great, thanks! We also use ArchitectureTest to test updates. It requires a message fuzzer to generate a list of random messages which could be applied sequentially and return new model. It would be nice to expose Animation.Msg fuzzer in order to generate random animation messages. Please let me know if it makes sense and if you would like me to do a PR for it?