Make hlist universe polymorphic.

This change makes it possible to use hlists in the proof mode, which
itself uses hlists in the implementation of the specialize tactic.

prelude/hlist.v

 From iris.prelude Require Import tactics.
+Local Set Universe Polymorphism.
 
 (* Not using [list Type] in order to avoid universe inconsistencies *)
 Inductive tlist := tnil : tlist | tcons : Type → tlist → tlist.