Rewrite the Horseshoe Gibbs sampler to use our rewrite system

[rlouf]

The Gibbs sampler for the horseshoe prior does not work on the conditional distributions found in the model, but on the conditional distributions of a transformed version of the model.

In particular, it exploits the fact that a half-Cauchy distribution can be intepreted as a mixture of inverse-gamma distributions, and that if $Z \sim \operatorname{InvGamma}(1, a)$ then $Z \sim 1 / \operatorname{Exp}(a)$.

We should instead implement these relations and update construct_samplers so it finds this structure and builds the sampling steps for each variable in the original model expression.

Related to #46, #65