fix(library/tactic): unfold tactic

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(frontends/lean): add unfold tactic command

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
refactor(kernel/instantiate): rename instantiate_params to instantiate_univ_params

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(frontends/lean): add repeat tactic command, refactor tactic sequence notation

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>