add several features to frozen distributions

Just some convenience wrappers. 

* add `a`, `b` attributes (support boundaries). 
* add `random_state` property
* add `expect` method.

All these were available with a bit of digging (if `f` is a frozen distribution, `f.dist` is a full-featured distribution instance, so that there always is `f.dist.a`, `f.dist.expect` etc), but it's IMO better to have them more easily discoverable.