feat(library/hidden_defs): hidden definitions are just hints for tactics and solvers

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(library/tactic): add unfold_tactic() that unfolds every non-hidden definition

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(library/tactic): add unfold_tactic() that unfolds every non-hidden definition