Make `unseal` TC opaque

This had a very slight positive impact on our development. But we don't have a lot of things sealed this way so the impact might be bigger in other developments.

stdpp/base.v

@@ -56,6 +56,7 @@ Add Search Blacklist "_unseal".
 Record seal {A} (f : A) := { unseal : A; seal_eq : unseal = f }.
 Global Arguments unseal {_ _} _ : assert.
 Global Arguments seal_eq {_ _} _ : assert.
+Global Typeclasses Opaque unseal.
 
 (** * Solving type class instances *)
 (** The tactic [tc_solve] is used to solve type class goals by invoking type